Preheat your oven to 350°F and lightly grease a 9x13-inch baking dish. Dice the onion into small, uniform pieces and cut both the red and green bell peppers into 1/4-inch pieces—uniform sizing ensures even cooking. Shred the Gruyère cheese fresh (you'll need 1.5 cups plus 0.5 cup reserved for topping), as freshly shredded cheese melts more evenly than pre-shredded. Cut the day-old biscuits into quarters or halves depending on their size; stale biscuits hold their structure better than fresh ones and won't turn to mush in the egg custard.

Heat a large skillet over medium-high heat and add the ground breakfast sausage, breaking it into small crumbles as it cooks for about 4-5 minutes until browned and cooked through. Remove the sausage to a plate and set aside. In the same skillet (keeping the rendered fat for flavor), add the diced onion and both bell peppers, sautéing for 3-4 minutes until softened but still slightly firm—you want them to retain some texture since they'll continue cooking in the oven. I like to keep the heat at medium so the vegetables don't brown too much and maintain their bright color.

Scatter the cut biscuit pieces evenly across the bottom of your prepared baking dish. Distribute the cooked sausage from Step 2 over the biscuits, then scatter the sautéed vegetables from Step 2 on top. Sprinkle the 1.5 cups of shredded Gruyère cheese over everything, making sure it's distributed fairly evenly. This layering method ensures every bite has sausage, vegetables, and cheese throughout the casserole.

In a bowl, whisk together the room-temperature eggs, half and half, kosher salt, and black pepper until well combined and smooth. I recommend using room temperature eggs because they incorporate more evenly with the cream and create a more uniform custard. Pour this mixture slowly and evenly over the assembled casserole, ensuring the liquid reaches all areas and soaks into the biscuits. Let the casserole sit at room temperature for at least 30 minutes (or up to 8 hours refrigerated) to allow the biscuits to absorb the custard—this resting period is crucial for a tender, cohesive casserole rather than a watery one.

Bake the casserole at 350°F for 35-40 minutes until the egg custard is mostly set but still has a slight jiggle in the center—it will continue to cook slightly as it cools. During the last 5 minutes of baking, sprinkle the reserved 0.5 cup of shredded Gruyère cheese evenly over the top and return to the oven to melt and lightly brown. The casserole is done when the edges are golden, the center is set, and a knife inserted near the middle comes out clean or with just a tiny bit of custard. Let it rest for 5 minutes before serving for easier portioning.