Preheat oven to 400 degrees F. Butter the bottom and sides of a casserole dish and set aside.

In 2 tbsp of butter, cook yellow onion and peppers until soft. Set aside.

Peel and slice potatoes very thin. A mandolin works perfectly for this. 

In a medium saucepan, over medium-high heat, melt remaining butter, add in flour and whisk frequently for 3-4 minutes or until the flour is fragrant and golden. Then, slowly pour in the cream, cumin and garlic powder and whisk until thickened. Remove from the heat and stir in two cups of cheese and mix until smooth. Stir in the salsa.

Divide the potatoes into three and layer a third of them on the bottom of the baking dish. Top with half the pepper mixture and green onions, spoon on 1/3 of the sauce and spread it around. Repeat once more, then finish with the final layer of potatoes, remaining sauce and sprinkle with remaining cheese.

Cover and bake for an an hour covered. Remove cover and place under the broiler for about five minutes or until the cheese turns golden and bubbly.