Preheat your oven on the low broiler setting.
Heat a large cast iron or other oven-safe skillet over medium-high heat and add the sausage (if you're using it). Cook the sausage until cooked through.
If not using sausage, add the olive oil (skip the olive oil if you have fat in the skillet with the sausage). Add the onion and garlic and saute until the onion is soft and the garlic is fragrant (about 5-6 minutes).
Add the spinach and mushrooms to the pan and toss them in the onion and garlic until the spinach wilts and the mushrooms begin to brown release their juices.
Crack the eggs into a bowl and whisk them together well with the salt and pepper.
Pour the eggs over the veggies in the hot skillet and turn off the heat. Sprinkle the crumbled goat cheese over top of the eggs.
Add the pan to the oven under the low broiler for about 8 minutes, or until the eggs appear set in the centre of the pan. If 8 minutes isn't enough, watch the pan carefully through the oven door and cook for an additional 1-2 minutes.
Remove the pan from the oven and let sit for about 4-5 minutes before serving. This gives the frittata a chance to set fully in the middle and the cheese to soften and melt.
Cut into 8 slices and serve with toast and/or fresh fruit, or store the slices in meal prep containers in the fridge to reheat for breakfasts throughout the week.