In a jar with a tight-fitting lid, combine 1/2 cup all-purpose flour, 1/2 cup milk, 1 tablespoon sugar, 2 teaspoons melted butter, and 1 large egg. Seal jar, and shake hard to blend.
Generously brush a 6-cup, nonstick muffin pan with melted butter. Pour batter, dividing evenly among the 6 cups. Place pan in a cold oven on the middle shelf.
Heat oven to 450 degrees F; bake popovers 15 minutes. Reduce heat to 350 degrees F, and bake 17 minutes more. Do not open door while baking. Remove pan from oven; pierce popovers to let out steam.
Rinse 1 pint fresh strawberries, and slice the fruit with a plastic knife. Transfer popovers to plates, garnish with sliced berries, and serve with jam, if desired.