Mini Spinach Frittatas | 1 tablespoon extra virgin olive oil; 1 small onion, finely diced; 1 clove garlic, minced; 2 cups packed fresh spinach, chopped or a thawed 10-oz. box of frozen chopped spinach; squeeze dried; 10 large eggs (organic and cage-free, if possible); ½ teaspoon salt; ¼ teaspoon pepper; 2 tablespoons chopped fresh herbs (such as parsley, thyme, rosemary or chives); ½ cup crumbled feta or goat cheese (optional)