Combine olive oil, fish sauce, pepper, salt, grated garlic, and ground sirloin; divide into 4 (3/4-inch-thick) patties. Heat a grill pan over high heat. Coat pan with cooking spray. Add patties and onion slices to pan; cook 3 minutes on each side or until desired degree of doneness. Spread 1 teaspoon canola mayonnaise on top and bottom of hamburger buns. Layer bottom half of each bun with 1 patty, 1 lettuce leaf, 2 thin tomato slices, one-fourth of onion, and top half of bun.