Heat oven to 375°F. Butter a 2-quart dish with the reserved butter.
Warm olive oil in a skillet over medium-high heat. Add mushrooms and shallot.
Cook until mushrooms brown and moisture evaporates, about 7 minutes. Stir in garlic and thyme.
Season lightly with salt and pepper. Remove from heat and cool slightly.
Simmer milk with unsalted butter in a saucepan. Sprinkle in cornmeal while whisking.
Cook, whisking, until thick and smooth, 3–5 minutes. Take off heat and rest 3 minutes.
Stir in Gruyère, baking powder, remaining salt, and pepper. Fold in parsley.
Whisk in beaten eggs gradually until glossy and well combined.
Fold in the mushroom mixture. Spread batter into the prepared dish.
Bake until puffed, set, and lightly golden, 30–35 minutes. Rest 10 minutes.
Shower with chives. Scoop and serve warm.
Notes
Try swapping Gruyère with sharp cheddar or fontina. For a heartier version, fold in crisp bacon or roasted corn. Leftovers reheat well: warm, covered, at 325°F for 10–15 minutes or microwave gently until hot.This recipe is an original creation inspired by classic Herbed Mushroom Spoon Bread Recipe flavors. All ingredient ratios and instructions are independently developed.