PREHEAT oven to 325°F. BUTTER eight (8 oz.) ramekins. PLACE bread on a large rimmed baking sheet and BAKE until golden brown and crisp, about 15-18 minutes. TRANSFER to a large bowl.
HEAT 1 Tbsp. oil in a large cast-iron skillet over medium heat. ADD sausage, smashing with the back of a spatula, and COOK, until golden brown, about 2-3 minutes. BREAK up sausage with spatula into ½” pieces and CONTINUE to cook, stirring frequently, until no longer pink. Using a slotted spoon, TRANSFER sausage to bowl with toasted bread.
HEAT remaining 2 Tbsp. oil in same skillet over medium-high heat. ARRANGE mushrooms in skillet in an even layer and COOK, until browned, about 4-5 minutes. STIR mushrooms and CONTINUE to cook until mostly browned, about 4 minutes longer. ADD 1 Tbsp. butter and CONTINUE to cook, tossing occasionally, until other side is browned, about 4-5 minutes. ADD shallot, garlic, rosemary and ¾ tsp. salt, and CONTINUE to cook, stirring occasionally, until fragrant but not browned, about 2 minutes longer. TRANSFER mushroom mixture to bowl with bread and sausage. GRATE the gruyere into the bowl and MIX to combine.
WHISK eggs, egg yolks, half-and-half, pepper and remaining 2 tsp. salt in a medium bowl until blended. POUR over bread mixture, STIR well to combine, and SOAK 5 minutes. DIVIDE custard mixture among prepared ramekins, filling each cup just to the rim.
TRANSFER to oven, and BAKE until golden brown, slightly puffed, bubbling at the edges and no longer wet in the center, about 30–35 minutes.