Add a tablespoon of butter to a large saute pan and melt it over a medium low heat.
Add in the onions, and cook for about 2 minutes, until just translucent. Add in the spinach, and cook until all of it is wilted. Add in the red peppers, more butter if needed, and saute for about a minute. Set aside.
Grease a muffin pan that holds 12 muffins. Divide the sauteed veggies between the tins.
Crack the 9 eggs in a large mixing bowl. Add in the salt and milk. Whisk together.
Divide half of the cheddar cheese between the muffin tins and then divide the egg mixture.
Bake at 375° F for 10 minutes. Remove from heat and divide the remaining cheddar, topping the quiches.
Place back in the oven and bake until the eggs are set. Test with a toothpick.