Melt butter in 4-qt saucepan over med heat. Add onions and saute until transparent. Stir in flour and salt. Cook 1 minute stirring constantly.
Add milk, stir to blend. Add tomatoes, juice and all. Bring to a low, steady simmer. Add uncooked pasta. Cover and continue to simmer gently for about 8 to 10 minutes, stirring occasionally. (Mixture will thicken as it is cooking.).
Remove from heat. Add cheese. Stir gently until cheese is melted. Let sit 5 minutes fefore serving.
Note: For a cheesier version put in a casserole dish and top with an additional 1/2 cup of cheddar cheese and broil until cheese is melted and just starting to brown.
For an Italian twist try substituting the Cheddar cheese with 1 Cup fontina cheese and 1/2 Cup of freshly grated parmesan cheese.