Sauteed fennel takes on a more mellow flavor that pairs nicely with tuna and capers in this main-course pasta dish.
Cook linguine according to package instructions. Drain and return to pot; reserve 1/2 cup pasta water.
Trim fennel bulbs; reserve 1/4 cup chopped fronds. Quarter, core, and thinly slice bulbs crosswise; cook in 1 tablespoon olive oil in a skillet over medium-high heat until golden, stirring occasionally, 10 minutes. Add to pasta along with fronds, lemon juice, capers, remaining 2 tablespoons olive oil, and reserved pasta water.
Season with salt and pepper. Flake in tuna. Gently toss.