Preheat oven to 350°. Drain spinach well, pressing between paper towels. Melt 2 Tbsp. butter in a large Dutch oven over medium heat. Add onion, and sauté 5 minutes or until tender. Add garlic, and sauté 1 minute; remove from heat.

Stir cream cheese into onion mixture until melted and well blended. Stir in spinach, milk, and next 3 ingredients. Stir together egg, drained pimiento, and 1 1/2 cups cheese; stir egg mixture into spinach mixture. Spoon mixture into lightly greased (with cooking spray) ramekins or a 2-qt. baking dish; sprinkle with salt and pepper.

Toss together panko, 2 Tbsp. melted butter, and remaining 1/2 cup Cheddar cheese; sprinkle over spinach mixture.

Bake at 350° for 50 minutes or until bubbly and golden.