Heat olive oil in a large nonstick frying pan over medium heat. Add onion, mushrooms, salt, and pepper, and stir often until onions are light golden, about 10 minutes.
Increase heat to medium-high, add sausage, and stir often until sausage has browned slightly and released some fat, about 5 minutes.
Meanwhile, in a medium bowl, whisk together eggs and 3 tbsp. water.
Reduce heat to medium and add garlic and chile flakes. Stir until garlic is fragrant, about 1 minute. Add spinach to pan and toss with tongs until it has wilted, about 2 minutes. Transfer sausage-vegetable mixture to a bowl and wipe pan clean.
Add eggs to pan and scramble just until set, 4 to 5 minutes, then gently stir in sausage-vegetable mixture. Serve with hot sauce if you like.
Note: Nutritional analysis is per serving.