Preheat oven to 350°F (175°C). Spray a 12-cup muffin tin generously with nonstick spray or use silicone liners. Don't skimp—eggs stick like gossip.
Heat 1 tbsp oil in a skillet over medium. Add sausage and cook, breaking into small crumbles, until browned and cooked through, 5–7 minutes. Drain excess fat if needed.
In the same pan, add onion and peppers with a pinch of salt. Cook 3–4 minutes until softened. Stir in garlic for 30 seconds. Add spinach and cook just until wilted, 1–2 minutes. Remove from heat.
In a large bowl, whisk eggs, milk, salt, pepper, and smoked paprika until well combined and slightly frothy.
Divide sausage and veggie mixture evenly among the 12 muffin wells. Sprinkle cheese if using.
Pour the egg mixture over fillings, filling each cup about 3/4 full. Give the pan a gentle shake to settle.
Bake 16–20 minutes, until set in the center and lightly puffed. They may dome and then deflate—totally normal, not a baking fail.
Let rest 5 minutes in the pan, then run a thin spatula around edges to release. Move to a rack to cool completely if storing.
Enjoy hot or at room temp. Add hot sauce, salsa, or a dollop of Greek yogurt if you're fancy.