1. Cut off narrow neck of each squash. Insert a small paring knife into cut ends of squash. Carefully twist knife to remove pulp. (Do not pierce sides of squash.)
2. Combine 2 teaspoons garlic, lamb, and next 7 ingredients (through pepper) in a bowl. Stuff lamb mixture evenly into squash to within 1/4 inch of opening. Place squash horizontally, slightly overlapping, in a 5-quart electric slow cooker coated with cooking spray.
3. Combine remaining 2 teaspoons garlic, onion, and next 3 ingredients (through tomatoes); pour over squash. Cover and cook on LOW for 6 hours or until squash is tender and filling is done. Serve squash with sauce; sprinkle with additional mint, if desired.