Instructions:
- Preheat oven to 375°F (190°C). Grease the 9x13 inch baking dish thoroughly.
- Brown the sausage in a skillet over medium heat, breaking it up. Drain excess fat, reserving about 1 tablespoon. Sauté the onion in the reserved fat until soft (3-4 minutes). Add garlic and cook for 1 minute more. Stir in Dijon mustard and set aside.
- In a large mixing bowl, whisk together the flour, baking powder, and salt.
- Cut the cold, cubed butter into the dry ingredients using a pastry blender or fingertips until the mixture resembles coarse crumbs.
- Gently stir in the 1 cup of milk until just combined to form a shaggy dough. Press this dough evenly into the bottom of the prepared baking dish to form the base layer.
- Sprinkle half of the shredded cheddar evenly over the dough base. Spread the cooked sausage/onion mixture over the cheese. Top with the remaining half of the cheddar.
- In a separate bowl, whisk the 8 eggs until pale and frothy. Gradually whisk in the 1 1/2 cups of milk/half-and-half, dried thyme, and black pepper until smooth.
- Slowly and evenly pour the egg custard mixture over the layered ingredients. Gently press down on any dry spots poking through the liquid.
- Let the casserole sit on the counter for 10-15 minutes to allow the base to absorb liquid.
- Bake for 45–55 minutes, or until the centre is set and the top is golden brown and puffy.
- Let the casserole rest on a wire rack for 10 minutes before slicing and serving.