2 1/4cupsgrated cheese like Havarti and/or Gruyère You can also sub in Swiss or Jarlsberg
2cup Panko bread crumbs
Preheat oven to 350°F. Butter a 9 X 11 inch casserole dish.
Melt butter over medium low heat in a medium saucepan. Whisk in the flour, whisking constantly for about 1 minute. Add salt and pepper.
Add sherry, whisking constantly for about 1 minute. Raise heat to medium. Whisk in the cream, whisking constantly until mixture beings to bubble and thicken. Add nutmeg. Remove from heat when mixture is the thickness of a cheese sauce. Set aside.
Vegetables & Cheese
Peel the potatoes, beet and onion. Cut them in half vertically and then slice thinly cross-wise. Shoot for slices to be of similar width. A mandolin on thick slice setting would work well here. Sprinkle the flour over the onion slices to coat.
Layer the vegetables in the buttered casserole dish starting with a layer of beet slices, then half the onion. Spread 2/3 cup of Béchamel over the slices. Then layer Yukon, sweet potato and then the remaining onion. Top with remaining béchamel sauce.
Distribute grated cheese evenly over the casserole.
Spray a sheet of foil with baking spray and loosely tent the casserole dish. Bake for 40 minutes covered. Do not pinch down the sides. You do not want the vegetables to steam.
While casserole is baking melt the butter for the topping. Add the salt and pepper. Toss in the panko crumbs and toss to coat.
Remove casserole from oven after the 40 minutes above. Distribute crumbs evenly over the top and return to the oven for another 20 minutes uncovered. Check if a fork can easily pierce the vegetables. Return to oven if needed. If crumbs are getting too dark lay the sheet of foil loosely over the top of the casserole again.
Remove from oven when vegetables are fork tender.
Note: Cooking time will depend on the actual size of your casserole and the thickness of your slices. Mine actually took one hour and 20 minutes to become fork tender. Don't skip the foil or your cheese and crumb topping may get too dark.