Preheat your oven to 375°F and generously spray a 12-cup muffin tin with cooking spray. Grandma always said, 'A well-greased pan is half the battle won!' I once tried to skip this step thinking my non-stick pan would be fine, and ended up with muffins stuck so badly I had to serve them in pieces. Never again!
In a large skillet over medium heat, cook and crumble the breakfast sausage until it's nicely browned and no pink remains, about 6-7 minutes. Drain the excess grease and let it cool slightly. Grandma taught me to save a little of that flavorful grease - just a teaspoon - to add back into the batter. 'That's where the flavor lives, Jazzy!' she'd say with a wink.
In a large mixing bowl, whisk together 6 eggs and milk until well combined. Don't overbeat - Grandma would count to thirty while whisking and then stop. She said over-beaten eggs make tough muffins. I learned this the hard way when I got distracted by a phone call and left the mixer running!
Add the Bisquick baking mix to the egg mixture and stir just until combined. Remember what Grandma always said: 'Mix it like you're folding in a cloud - gentle and patient.' The batter should still have a few small lumps. If you mix until it's perfectly smooth, your muffins will be dense instead of fluffy.
Gently fold in the cooked sausage, sharp cheddar cheese, green onions if using, and black pepper. This is where Grandma's secret patience trick comes in - let the batter rest for 5 minutes after mixing. 'The batter needs to wake up,' she'd say. This little wait makes the muffins incredibly tender.
Using a 1/3 cup measure or ice cream scoop, divide the batter evenly among the 12 muffin cups. They should be about 3/4 full. Give the pan a gentle tap on the counter to remove any air bubbles - another one of Grandma's tricks for even baking.
Bake for 18-20 minutes, or until the muffins are golden brown on top and a toothpick inserted in the center comes out clean. The muffins will puff up dramatically while baking - don't worry, that's normal! They'll settle a bit as they cool.
Let the muffins cool in the pan for 5 minutes before removing them to a wire rack. This resting time helps them set properly. Grandma always said, 'Good things come to those who wait, but not too long when breakfast is involved!' Serve warm and enjoy watching them disappear!