Preheat your oven to 375°F and spray a 9-inch pie plate with cooking spray. Remove the pie crust from the refrigerator and gently press it into the prepared pie plate, allowing it to come up the sides naturally. This is your foundation, so handle it gently to avoid tears. Set aside while you prepare the filling.

Heat the avocado oil in a large skillet over medium-high heat. Add the crumbled breakfast sausage and cook for 5-7 minutes, breaking it into small pieces with a spoon as it cooks, until it's golden brown and cooked through. Remove the sausage to a plate, leaving about 1 tablespoon of the rendered fat in the pan. Reduce heat to medium, add the diced onion, and cook for 15-20 minutes, stirring occasionally, until deeply caramelized and sweet—this long cooking develops incredible flavor that will anchor your quiche. I find that low and slow caramelization of onions makes all the difference in quiche depth.

Add the diced red and green bell peppers to the caramelized onions and cook for 4-5 minutes, stirring frequently, until they're tender but still have a slight bite. Add the minced garlic and cook for another 1-2 minutes until fragrant. Remove the entire mixture from the pan and set aside to cool slightly—this prevents the hot vegetables from partially cooking the eggs when combined.

In a large mixing bowl, whisk together the eggs, whole milk, and heavy cream until well combined and slightly frothy. Stir in the sage, kosher salt, and freshly ground black pepper. Add the cooked sausage from Step 2, the cooked vegetable mixture from Step 3, and the shredded cheddar cheese. Fold everything together gently but thoroughly, making sure the cheese and sausage are evenly distributed throughout. I like using Kerrygold butter in the cheese for richness, though you won't use butter here—it's already in the cheddar—but the quality of your cheese really matters in a quiche.

Pour the custard filling from Step 4 into the prepared pie crust, spreading it evenly. Place the pie plate in the preheated 375°F oven and bake for 45-55 minutes, until the top is golden brown and a knife inserted near the center comes out with just a few moist crumbs (not wet, but not completely set). The quiche will continue to cook slightly as it cools, so don't overbake or it will become rubbery.

Remove the quiche from the oven and let it rest at room temperature for 10 minutes before slicing. This resting period allows the custard to set completely and makes for cleaner slices. Serve warm or at room temperature.