Red, White, and Green Grilled Cheese

So good, your children might not even notice the “green stuff”

1 tsp garlic, minced (about ½ clove)
1 small onion, minced (about ½ cup)
2 C frozen cut spinach, thawed and drained (or substitute 2 bags (10 oz each) fresh leaf spinach,
rinsed)
¼ tsp ground black pepper
8 slices whole-wheat bread
1 medium tomato, rinsed, cut into 4 slices
1 C shredded part-skim mozzarella cheese
Nonstick cooking spray