I WAS at Nonna and I had to pee, as one typically does after a couple of waters and a cocktail. I wound my way toward the loo, passing people packed into every cozy table and filling the horseshoe-shaped bar. It's a dark den, dimly lit by low lights and mason jar candles, and I arrived at a heavy red velvet curtain. I pushed past, only to hit another curtain. I popped my head through, and immediately five heads enjoying their dinners at DOC swiveled to stare. I was exposed. And I was reminded—not for the first time that night—that I was at the junior restaurant.

Nonna is the more casual sister tavern for Italian fine-dining stalwart DOC—so much so that they share the aforementioned curtained door. This has many advantages. Among them is the approachable price point, with the most expensive dish just clearing $20. Also nice is the ability to roll in wearing a Patagonia puffy jacket and not get the side-eye from a skinny chick in a designer romper two tables over. Nonna is a welcome addition to the already bursting blocks in Concordia that include Beast, Expatriate, Wilder, and Yakuza (which, like DOC and Nonna, is owned by Dayna McErlean).

Nonna's chef, Jobie Bailey, is also imported from the other side of that red velvet curtain, and he's done a fine job translating the refined plates of DOC to rustic seasonal fare. A simple grilled focaccia in a puddle of spicy olive oil ($3) was absurdly tasty, and the only dish I ordered twice. A few slices also appear with the fattest mussels I've ever seen, served with gremolata in a classic white wine butter sauce ($14). In fact, we could have used a bit more bread—I'm ashamed to say I used an empty shell as a spoon to get at more of the artery-clogging goodness. A nettle and pistachio pesto was a welcome harbinger of spring, blended with a small bird's nest of linguini ($13).

The tequila radler—with Hornitos, grapefruit radler, and a splash of Campari ($9)—is a crisp, bitter drink sweetened by the agave liquor that I’ve declared my sip of the summer. The Meyer Lemon Frisco Sour ($9) marks the arrival of a rare cocktail bird, a take on a lesser-known classic served here with Dickel rye, Bénédictine, and Meyer lemon.

But there are ways in which Nonna’s laidback approach backfires, namely in the service department. Two visits with medium-sized groups brought us the same waiter, who both times largely abandoned us after taking our dinner order (and with drinks this good, it’s a shame to work so hard for a second one), and forgot key dishes. On our first visit, we were missing an entrée, and the check arrived without an offer of dessert. The bar manager handled the bungle graciously, and we gobbled down the espresso panna cotta, a creamy, balanced take on the dish that made my morning latte seem downright drab. On the second visit, our table of five was shorted two appetizers. We were offered dessert, but then no one returned to take our order. We gave up and left. This must change, Ms. McErlean.

A few dishes failed. Most notably the charred rapini with chili flake ($7), which a former chef among us described with the industry term "hammered." It was left behind after a few tremulous bites. And I'm starting to think it's just me and farro—I can't remember the last time I had this increasingly popular ancient grain served in a way I've found enjoyable—but its blending here with mushy spaghetti squash and too few mushrooms ($12) tasted salty and strangely like Rice-A-Roni.

When in doubt, order something that's served in a miniature cast-iron dish. Both the vegetarian tomato ricotta manicotti ($14) and the crisp whole trout topped with salmoriglio (think salsa meets pesto), $15, were among the best offerings by far.

Though Nonna means "grandma" in Italian, it really is more of a little sister to DOC—allowed to dress down and get playful—and, once it gets out of those gawky stages, it's the kind of kid who could shine.

Tues-Thurs 5-10 pm, Fri-Sat 5-11 pm. Wine, a few beers, and full bar. Reservations accepted for six or more. Back patio will open in summer months.