Ham and cheese egg muffins (crustless mini quiches)

Ingredients

• 1 small yellow onion, finely chopped
• Olive oil
• 1 tablespoon (15 ml) balsamic vinegar
• 2 tablespoons (30 ml) butter
• 227 g white mushrooms, thinly sliced
• 12 eggs
• 1 cup (250 ml) grated sharp cheddar cheese
• 2/3 cup (160 ml) smoked ham, diced
• 1/3 cup (80 ml) milk
• 2 tablespoons (30 ml) chopped fresh parsley
• 1/2 teaspoon (2.5 ml) Italian spices
• 1/2 teaspoon (2.5 ml) baking powder