Sauté crab meat and onion in 1 tablespoon butter until crab meat is heated and onion is tender; set aside.
Melt 1/2 cup butter in top of a double boiler over boiling water. Gradually add yolks, stirring constantly, until slightly thickened. Stir in lemon juice, salt, and pepper. Fold reserved crabmeat- onion mixture into sauce.
Spoon mixture into four 6- ounce custard cups; sprinkle with cheese. Bake at 375° for 10 minutes or until lightly browned. Serve immediately.