Preheat oven to 375°F. Coat a 12-cup muffin tin with cooking spray.
Heat oil in a large nonstick skillet over medium-high heat. Add sausage; cook, stirring often and breaking into crumbles, until browned and cooked through, about 4 minutes. Using a slotted spoon, transfer the sausage to a paper-towel-lined plate, leaving the drippings in the pan. Reduce heat to medium; add mushrooms and cook, stirring occasionally, until softened and browned in most spots, 6 to 8 minutes. Add spinach; cook, stirring constantly, until wilted, about 30 seconds. Transfer the vegetable mixture to the plate with the sausage. Let cool for 5 minutes.
Meanwhile, whisk eggs, milk, pepper, garlic powder, onion powder and salt in a large bowl until thoroughly blended. Stir in the cooled sausage and mushroom mixture. Divide the mixture among the prepared muffin cups, about ¼ cup each.
Bake, rotating the pan front to back once, until puffed and set, 14 to 16 minutes. Let stand for 5 minutes before removing the egg bites from the pan.
