Heat oven to 400°F.
Cook rice as directed.
In a small saucepan, melt butter over medium heat, sauté garlic and pepper flakes until garlic is lightly golden, 1 minute. Add vinegar, orange juice and soy sauce; cook until bubbling, 3 minutes.
In a bowl, combine cornstarch and 1 teaspoon water; stir into garlic glaze. Cook until slightly thick, 1 minute.
Arrange fillets on a foil-lined baking sheet. Spread 1 teaspoon glaze over each fillet. Roast until just cooked through, 12 minutes.
In a medium skillet, warm oil over medium heat. Add 1 package spinach; sauté until just wilted. Remove from pan; set aside. Repeat with second package spinach.
Divide rice among 4 bowls; top each with 1/4 spinach and 1 salmon fillet, broken into large pieces.
Drizzle with remaining sauce; sprinkle each with 1/2 teaspoon sesame seeds.