PREHEAT OVEN TO 400 F and place rack at center. Line 9" x 13" quarter-sheet pan with foil and spray with cooking spray or brush with oil.
TOSS TOGETHER THE VEGETABLES with ½ teaspoon each salt and pepper, the crushed red pepper, the Herbes de Provence, and a tablespoon of olive oil in a large bowl. Set aside.
PLACE SALMON FILLETS SKIN SIDE DOWN IN THE CENTER OF THE SHEET PAN two inches apart and spread prepared vegetables around and between them. Drizzle salmon with a little olive oil, sprinkle with salt and pepper, and top with 3-4 lemon slices each. (Extra lemon can be added to the vegetables, if desired.)
ROAST FISH AND VEGETABLES for 15 minutes or until fish is done to your liking. I’m fond of salmon cooked to medium (about 130 degrees F on an instant read thermometer) but do let it cook another few minutes if you like it well-done. Move pan to a cooling rack for 5 minutes and serve hot or warm with Horseradish Mustard sauce, if desired.