The General Recipe

Preheat and prepare

Preheat oven to 375°F (190°C). Grease an 8-inch square baking pan or line with parchment and set aside.

Combine dry ingredients

In a medium bowl whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until evenly combined.

Cut in butter

Cut the cold unsalted butter into small pieces and use a pastry cutter or two forks to work it into the dry mixture until the texture resembles coarse crumbs with some pea-sized bits of butter remaining.

Mix wet ingredients

In a separate bowl whisk together the whole milk, sour cream, large egg, and vanilla extract until smooth and homogenous.

Combine wet and dry

Pour the wet mixture into the flour-butter mixture. Stir gently with a spatula until just combined — do not overmix. The batter will be thick and slightly lumpy.

Prepare cheese topping

Fold about half of the grated gruyère and half of the cheddar into the batter gently, leaving the rest for the top. Stir in the Dijon mustard and half of the sliced green onions so they are distributed through the batter.

Transfer and top

Spoon the batter into the prepared pan and smooth the top with a spatula. Sprinkle the remaining gruyère and cheddar evenly over the surface. Scatter the remaining sliced green onions on top, then dust with black pepper and smoked paprika for color and a subtle smoky finish.

Bake

Bake in the preheated oven for 20–25 minutes, or until the top is golden brown and a toothpick inserted into the center comes out with just a few moist crumbs. If the cheese browns too quickly, loosely tent with foil for the last 5–7 minutes.

Finish

Remove from the oven and brush the top lightly with the tablespoon of melted butter (optional) for shine and flavor. Let cool in the pan for 10 minutes to set.

Slice into 9 squares and garnish with microgreens or chopped parsley. Serve warm with a few drops of hot sauce on the side if you like a touch of heat.

Storage: Leftovers keep refrigerated in an airtight container for up to 3 days. Reheat in a 350°F (175°C) oven for 8–10 minutes to restore crispness.