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; saut 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.