Preheat an oven to 375 degrees. Butter a 2 quart casserole. Warm 1 1/2 cups of the milk in a saucepan. Combine remaining milk and flour in a jar and shake. Add to hot milk and whisk until smooth and slightly thickened. Remove from heat and stir in the shredded cheese. Add salt and pepper and garlic powder
Very thinly slice the potatoes. Layer half of the potatoes, then half of the onions, poblano, and jalapeño. Layer remaining potatoes, onions and peppers. Pour slightly cooled cheese sauce over the potatoes making sure to cover all ingredients. Sprinkle paprika on top.
Cover and bake for 1 hour. Uncover and bake until potatoes are tender and the top is browned approximately 30 minutes.