Heat oven to 425°.
Sprinkle ribs lightly with salt and pepper; arrange in a large, shallow roasting pan, meaty side up. Bake, uncovered, for 30 minutes. Meanwhile, in a saucepan, combine the ketchup, Worcestershire sauce, lemon juice, chili powder, 1 teaspoon salt, and 1/2 teaspoon pepper. Bring the mixture to a simmer, stirring constantly. Remove from heat.
Reduce the oven temperature to 350°. Remove the ribs from the oven and drain off excess grease from the pan. Brush the ribs generously with the sauce mixture and arrange onion slices over the ribs. Return to the oven and bake for 1 1/2 hours, brushing with the sauce several times.
Calories per serving: 650
Fat per serving: 48.9g