Crack the eggs into a medium bowl. Add the cup of whole milk, 1/2 teaspoon of the salt, and 1/2 teaspoon of the black pepper. Whisk vigorously until blended and slightly frothy. Finely dice the onion, mince the garlic, chop the parsley and chives (if using). Measure out the cheese if using.

Cook sausage

Heat a large nonstick or stainless-steel skillet over medium-high heat. Add 1 tablespoon olive oil. When hot, add the ground pork sausage, breaking it into small pieces with a spatula. Cook for 6–8 minutes, stirring occasionally, until browned and no longer pink. Use the spatula to crumble it to bite-sized pieces. If the sausage releases excess fat, spoon off and discard all but about 1 tablespoon of fat from the pan (leave some for flavor).

Sauté aromatics

Push the cooked sausage to one side of the pan or transfer briefly to a bowl, add 2 tablespoons butter to the cleared pan over medium heat. Add the diced onion and sauté for 3–4 minutes until translucent and slightly golden. Add the minced garlic and cook 30–45 seconds until fragrant, stirring frequently. Return the sausage to the pan and stir to combine with the onions and garlic.

Season and combine

Reduce heat to medium-low. Sprinkle in the remaining 1/2 teaspoon salt, 1/2 teaspoon black pepper, and red pepper flakes (if using). Stir to distribute seasonings evenly through the sausage mixture.

Add eggs

Pour the whisked egg and milk mixture evenly over the sausage-onion mixture in the pan. Let sit undisturbed for 20–30 seconds to begin setting at the edges. Using a heatproof spatula, gently push the cooked edges toward the center, allowing uncooked egg to flow underneath. Continue folding gently every 20–30 seconds until eggs are mostly set but still moist — about 2–4 minutes. Avoid over-stirring to keep eggs soft.

Finish and melt cheese

When eggs are nearly done, sprinkle the shredded cheddar cheese evenly over the top if using. Cover the pan with a lid for 30–60 seconds to melt the cheese and finish cooking the eggs through. Remove from heat promptly when eggs are just set; residual heat will finish them.

Garnish and serve

Sprinkle chopped parsley and chives over the Sausage & Egg Mix. Taste and adjust seasoning with additional salt and pepper if needed. Serve immediately: as a filling for breakfast sandwiches, on toast, with warm tortillas, or alongside roasted potatoes or a simple salad.

Make-ahead and storage

Cool completely and refrigerate in an airtight container for up to 3 days. Reheat gently in a skillet over low heat or in the microwave in 30-second bursts, stirring between, until warmed through. Do not freeze with dairy/milk included for best texture; you can freeze cooked sausage separately for up to 3 months.