Preheat oven to 350 degrees. Butter muffin tin.
Using a mandolin or sharp knife, thinly slice potatoes and onions.
In a large skillet, heat olive oil and 1 tsp. butter; add onions and fresh herbs. Saute until onions are tender.
In a medium sauce pan, melt 3 Tbl. butter over , then add flour and whisk to combine. Let sit until mixture is bubbly. Add cold milk and whisk to combine. Bring to a slow boil over medium heat, then reduce heat stirring occasionally. Combine two shredded cheeses and add 1 cup to sauce mixture. Stir until melted. Remove from heat.
Layer potato slice, sweet potato slice, potato slice, spoonful of herbed onions, spoonful of cheese sauce, then repeat as many times as your tin will hold. Fill each well fully as they will reduce when cooking. Sprinkle tops with remaining shredded cheese.
Bake uncovered at 350 for 45 minutes. If potatoes become very brown, cover with foil and continue cooking until potatoes are tender. Let stand covered for 10 minutes before serving.