Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with aluminum foil.
Brush insides of the acorn squash with melted butter. Dust with garlic powder. Place acorn squash halves cut-side up on the baking sheet.
Roast in the preheated oven until tender, 50 to 60 minutes.
Heat oil in a large skillet over medium-high heat. Cook red bell pepper until tender, about 4 minutes. Stir in onion and cook until softened, 3 to 4 minutes. Reduce heat to medium and add garlic; cook until fragrant, about 2 minutes. Stir in spinach until it turns dark green, about 3 minutes.
Stir bread crumbs into the skillet; cook until lightly toasted, 2 to 3 minutes. Remove from heat; add Parmesan cheese. Mix in feta cheese thoroughly. Season mixture with salt and pepper.
Fill acorn squash halves with the red bell pepper mixture, mounding it slightly on top.
Bake in the preheated oven until evenly browned, 20 to 25 minutes. Cool for 5 minutes before serving.