Preheat oven to 350° F.
Place diced potatoes in a medium saucepan. Add water until potatoes are just covered. Add ½ tsp salt. Boil for about 15 minutes or until potatoes are just fork tender.
Meanwhile, melt first amount of butter in a cast iron casserole or heavy oven safe saucepan. Add onions and sauté. Stir in flour and allow it to cook for a minute. Slowly add evaporated milk so no lumps form. Add salt and pepper.
When potatoes are finished boiling, drain, reserving about one cup of the water and add to the sauce. Stir. If you find the sauce a little thick, add more water or milk until you get the consistency you want.
Stir in cooked potatoes, carrots, bell pepper and lobster.
Sprinkle shredded cheese evenly over top.
In a 1-cup glass measure, melt second amount of butter. Add fine breadcrumbs and stir with a fork. Sprinkle over top of cheese.
Bake in a 350° F oven for about 30 minutes or until golden brown and bubbly.