Add the oil/ butter in a heavy bottom saucepan on a medium flame. Add chopped onions, jalapeno peppers, and minced garlic. Sauté until cooked.
Add milk and heavy cream/half and half. Bring it to a mild boil and add flour. Use a whisk to stir it continuously to avoid lumps. Let the milk thicken now.
Add cheese once the milk starts thickening up. Use a whisk to stir until the cheese melts. Add salt.
Serve to top it with tomatoes and scallions.