Rinse capers in a sieve and transfer to a small bowl. Cover capers with water by about 2 inches and soak 30 minutes. Drain, then repeat soaking.

Drain and rinse capers, then squeeze dry and coarsely chop.

Cook onion, cut sides down, and garlic in oil in a 4- to 5-quart heavy pot over moderate heat, stirring occasionally, 3 minutes. Add capers and cook, stirring occasionally, 5 minutes. Remove and discard garlic and onion. Add canned and cherry tomatoes and cook, stirring occasionally, 3 minutes. Stir in mint and pepper, then reduce heat to low and simmer, covered, stirring occasionally, 1 1/2 hours.

Shortly before sauce is done, cook pasta in a 6- to 8-quart pot of boiling salted water until al dente. Reserve about 1 cup cooking water, then drain pasta. Add pasta to sauce and cook over high heat, tossing, 10 to 15 seconds. If pasta seems dry, add a few tablespoons reserved cooking water.

Cooks' note:
Sauce can be made 3 days ahead and chilled, covered.