When a chef and musician swap lessons, one learns how to grill a juicy leg of lamb, the other how to play guitar for a post-dinner jam.

Berkeley, California, has a reputation as a '60s holdout, with a communal ethic that means neighbors feel free to pick lemons from each others' trees without necessarily asking for permission. "Urban foraging" is what Michael "Cal" Peternell, a chef at Chez Panisse Café, calls it. When Cal and a friend, Mike Marshall, began bartering lessons with each other a couple of years ago—Cal has been teaching Mike how to cook and Mike has been teaching Cal how to play guitar—it was yet another expression of that friendly Berkeley give-and-take. One that put those "foraged" lemons to excellent use.

Mike, who is famous for his virtuosic mandolin, guitar and violin recordings, both solo and with musicians like Béla Fleck, is an avid home cook eager to improve his technique (he owns 200 cookbooks). Cal, meanwhile, wanted to learn guitar. When Cal's 11-year-old son Hopper started taking fiddle lessons with Mike's wife, Kaila Flexer, Cal and Mike struck a deal to teach each other. "It's one of those arrangements where each of us feels he's getting the better end of the bargain," Mike says.

When Mike isn't on tour, he and his family visit Cal's house on Wednesday afternoons. Hopper might learn some new Balkan tunes, while Cal works on the chords to Leadbelly's "In the Pines." Afterward, the men head to what Mike describes as Cal's "funky little kitchen," where the stove is on a slant, the butcher-block countertop is well worn, and the open back door lets in a breeze.

For a lesson on preparing Easter dinner, Cal bought legs of lamb from the Berkeley Bowl Marketplace and snipped rosemary from the communal garden behind his house. He and Mike were planning to marinate them in garlic and rosemary and grill them until they became deeply crusted and succulent. Cal also scavenged Meyer lemons from his neighbor's tree for a sweet and slightly bitter upside-down cake topped with thin slices of the fruit with the rind still on.

"Most of the stuff Cal shows me wouldn't appear in a cookbook," Mike says. Cal teaches what he calls "mother recipes"—ones that are flexible enough to accommodate all kinds of ingredient substitutions. For example, the rosemary-flavored marinade for the lamb would work well with almost any other meat (except perhaps pork, which Cal thinks pairs better with sage). Later, Cal explained why he throws huge pinches of salt into the water when he's blanching cauliflower to add to his beet salad. Taste the water, he advises; there should be enough salt so that you can tell it's there, but not so much that the water tastes like the sea. The Chez Panisse cooks, he adds, always keep deep bowls of salt on hand, which they've nicknamed "the Flavor Enhancer."

Around 7 p.m. the Marshall and Peternell families sat down to taste the results of the lesson. After dinner, Mike brought out his mandolin, Cal his guitar, and Kaila and Hopper their fiddles. These are the only times Hopper is willing to play with Cal anymore. "He surpassed me so quickly that he's usually embarrassed to play with me," Cal says. But if Hopper ever decides to learn how to cook, he knows where he can get lessons for a song.