Heat a skillet over medium heat. Add 1 tablespoon of butter, the onions and peppers to the pan. Cook until they begin to soften. Add the garlic, sausage and sage. When the sausage is cooked through remove from heat and cool completely.
Butter a casserole dish with 1 tablespoon of butter. Cut or tear the biscuits into pieces. Mix the biscuits with the cooled sausage mixture. Place 1/2 the biscuit mix into the casserole. Top with half the cheese. Then layer with the rest of the biscuit mix and cheese.
Whisk together eggs, half and half, yogurt, dijon, salt, and pepper. Pour over the biscuit mixture. Press gently to make sure everything is distributed evenly. Heat the oven to 350 degrees. Place the casserole in a deep sided pan. Place the dish in the oven and fill the outer pan with hot water 1/2 way up the sides of the casserole. Bake for 1- 1 1/4 hours. The top should be brown and firm to the touch. There should be no jiggly spots. Remove from the oven and cool for 15 minutes before serving.