Rinse and drain broccoli rabe; cut diagonally into 2-inch lengths. Rinse and drain radicchio; core and cut head lengthwise into 1/2-inch-thick slices. Rinse and dry lemon; cut in half lengthwise, then thinly slice crosswise, discarding ends and seeds.
In a 12- by 17-inch baking pan, mix broccoli rabe, radicchio, lemon, olive oil, pancetta, and 1/4 teaspoon salt. Transfer half the mixture into another 12- by 17-inch baking pan and spread mixture in both pans level.
Roast in a 400° regular or convection oven until broccoli rabe is tender when pierced and radicchio is wilted, about 10 minutes. Add salt and pepper to taste.