On a cutting board, chop and reserve 1/4 cup fennel fronds. Trim the bulbs, cut them in half and very thinly slice by hand or with a mandoline. Transfer to a large serving bowl and add the sweet peppers, radicchio, lemon juice, a healthy douse of extra-virgin olive oil, the reserved fennel fronds, and salt and pepper, to taste. Toss and let stand for 15 minutes, then toss again and serve.