Over low to moderate heat, saute onion, garlic, and your vegetable of choice in butter. (If you use tuna packed in oil, you can drain the oil and use that to saute the onions.)
Season with salt and curry powder and continue to saute until veggies are tender.
Stir flour into vegetables until it coats them and combines with the butter/oil.
Over moderate heat, add enough of the milk to cover the vegetables, and stir to create a smooth sauce. As the milk begins to heat, the butter/flour mixture that was coating the vegetables will begin to blend with and thicken the milk. Once it becomes thick, add half of the remaining milk, and continue stirring until it again heats to the thickening point. Then add the rest of the milk, stir again, and allow to come to the thickening point for the last time. (This process is a bit difficult to describe, but not hard to do. Adding the milk at little at a time and stirring to keep it smooth as you go keeps the sauce from forming lumps and prevents scorching. You can also do this step over low heat, in which case you don't have to stand right over the pot stirring constantly, but it's still good to add the milk in stages, and stir regularly.).
Once the sauce is heated, add the tuna, and bring back to simmering. If the tuna is packed in oil, drain the oil (which you may have used in step one). If it's packed in water, you can just dump the whole can into the sauce, water and all.
Serve tuna curry over rice. A green veggies side is nice (steamed asparagus or broccoli, cucumbers in vinaigrette, green salad, or peas).