Place pepper on a foil-lined baking sheet; broil 6 to 8 minutes or until blackened, turning occasionally. Place pepper in a zip-top plastic bag. Seal and let stand 15 minutes. Peel pepper and cut in half lengthwise. Discard seeds and membranes, and chop pepper.
Reduce oven temperature to 425°.
Rub sides and bottom of a 2 1/2-quart gratin dish with garlic; discard garlic. Coat sides of dish with cooking spray.
Slice potatoes crosswise into 1/8-inch-thick rounds. Arrange half of potato slices in bottom of prepared dish; top with half of chopped poblano. Drizzle 1 tablespoon butter over potato mixture and sprinkle with 1/4 teaspoon salt and 1/8 teaspoon pepper. Sprinkle with 1/2 cup cheese. Layer remaining potato slices on top of cheese, and top with 1 tablespoon butter, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Sprinkle with 1/2 cup cheese.
Place milk in a 4-cup glass measure. Microwave at HIGH 2 minutes or until gently boiling. Pour milk over potato mixture.
Bake at 425° for 45 minutes or until mixture is bubbly and potatoes are tender.