Preheat oven to 325°F. Combine garlic and pepper; press evenly onto all surfaces of beef Top Sirloin Petite Roast.
Place roast on rack in shallow roasting pan. Insert ovenproof meat thermometer so tip is centered in thickest part of beef. Toss onion with oil; place around roast. Do not add water or cover. Roast in 325°F oven 75 to 95 minutes for medium rare to medium doneness.
Remove roast when instant-read thermometer registers 135°F for medium rare; 150°F for medium doneness. Transfer roast to carving board; tent loosely with aluminum foil. Let stand 10 minutes. (Temperature will continue to rise about 10°F to reach 145°F for medium rare; 160°F for medium.)
Meanwhile, increase oven temperature to 425°F. Return onions to oven; roast an additional 15 minutes or until golden brown.
Carve roast into slices. Spread 1 tablespoon blue cheese dressing on bottom half of each. Top evenly with spinach, bacon, beef and onions. Close sandwiches.
Nutrition information per serving: 526 Calories; 180 kcal Calories from fat; 20 g Total Fat; 5 g Saturated Fat; 9 g Monounsaturated Fat; 94 mg Cholesterol; 1145 mg Sodium; 49 g Total Carbohydrate; 2.4 g Dietary Fiber; 39 g Protein; 5.9 mg Iron; 12.1 mg Niacin; 0.7 mg Vitamin B6; 3.5 mcg Vitamin B12; 7.4 mg Zinc; 58.2 mcg Selenium; 35.1 mg Choline;