2 tablespoons unsalted butter (1/4 stick), plus extra for coating the baking dish
6 scallions, ends trimmed and white and light green parts thinly sliced
1/2 medium yellow onion, finely chopped
3 medium garlic cloves, finely chopped
2 1/2 teaspoons kosher salt
1/2 teaspoon freshly ground black pepper
1 3/4 cups long-grain rice
3 cups low-sodium chicken broth
1 cup heavy cream
1 1/2 cups shredded extra-sharp white cheddar cheese