Preheat the oven to 400°F. Crush the cornflakes between two sheets of wax paper. Mix the crushed cornflakes with the thyme and pepper.
Rinse the chicken thoroughly and pat dry with paper towels. Place the chicken, breast side up, in a 13 x 9 x 2-inch baking pan. Brush the chicken with the melted margarine. Sprinkle the corn-flakes on top of the chicken breasts.
Cover the baking pan with a lid or a large sheet of foil tucked under the sides of the pan. Bake for 30 minutes. Carefully remove foil, then bake, uncovered, for another 30 minutes.