Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add butter; swirl pan until butter melts. Add onion and garlic; sauté 3 minutes or until tender. Add half of the spinach; cook 2 minutes or until spinach begins to wilt, stirring frequently. Add remaining half of spinach; cook 2 minutes or until spinach wilts, stirring frequently. Remove pan from heat.
Place flour in a small bowl; gradually add milk, stirring with a whisk. Stir flour mixture into spinach mixture; add salt, pepper, and nutmeg. Cook over medium-high heat 3 minutes or until thick and bubbly, stirring constantly. Remove from heat. Add cheese, stirring until cheese melts. Divide mixture evenly among 4 (4-ounce) ramekins or custard cups coated with cooking spray. Place ramekins on a baking sheet.
Place bread in a food processor; process until coarse crumbs measure 1/2 cup. Add parsley; pulse 3 times or until combined. Sprinkle about 2 tablespoons breadcrumb mixture over each serving. Broil 1 minute or until browned.