Preheat oven to 375 degree. Butter 13x9 casserole dish
Bring large pot of salted water to boil. Carefully add cauliflower florets
Cook 7-8 minutes until fork tender. Drain well
Rinse cannelli beans and add to bottom of casserole dish
Add drained cauliflower to top of cannelli beans
Meanwhile in medium sauce pan melt butter on low heat
Once butter is melted stir in cornstarch until thickened slightly into roux
Add mustard, garlic powder, cayenne pepper, salt & pepper
Stir in half & half and continue stirring until mixture thickens slightly
Add greek yogurt and continue stirring a few more minutes
Add ¾ cup of the grated cheese mixture(montery jack & cheddar) and continue cooking till cheese is melted
Remove sauce from heat and pour over the cauliflower and cannelli beans
Mix gently to coat cauliflower and beans
Top with freshly grated parmesan cheese and fresh dill
Bake in preheated oven 35-40 minutes until bubbly and slightly browned on top
Allow to sit 10 minutes prior to serving