Heat a skillet over medium heat and sear sausage. Transfer to a paper-towel lined plate.
In the same pan, add butter, squash and onions to the skillet and cook until soft, about 6 minutes. Stir eggs and milk together and pour over vegetables, stirring to cook through. Add sausage and sprinkle with cheese.
Microwave tortillas 15 seconds until warmed through. Divide egg mixture among tacos and serve. Garnish with cilantro, if desired.