5 Tbsp flour 
2 cups half-and-half
2 cups low-sodium chicken broth
1 ½ cups freshly grated sharp cheddar (you may want more)
salt and pepper to taste
1/2 tsp nutmeg  (to taste) 

50 (approx.) medium-sized boiling onions (NOT pearl onions)   

First, peel all the onions.  

Butter a big enough casserole dish to hold them all with sauce. 

Melt butter slowly in heavy saucepan. Add flour slowly and mix to make a roux. Whisk in the half-and-half. Let this cook slowly and whisk frequently until it gets to a white sauce consistency (not real thick because you'll be adding cheese). Stir in salt, pepper and nutmeg, and turn off heat.  

Bring a large pot of water to boiling. Add onions. Cook until they have some give but are not real tender. Drain. Put into buttered casserole dish.  

Heat up the white sauce. Add cheddar (more if you want it real cheesy). Whisk while the sauce thickens until really creamy. Taste for salt, pepper and nutmeg. You may also add a little pre-grated Parmesan, your choice.

Pour sauce over onions. Add breadcrumbs on top if you like. 

Bake at 350 until nice and bubbly on edges, at least 45 minutes. 

You may make the cheese sauce the day before and heat it slowly while prepping onions.