Prepare rice according to package, preheat oven to 375°F.
Place tuna and onions in large mixing bowl. Squish plum tomatoes in your hand to yield smaller pieces of them. Once done, mix tuna, onions and tomatoes together.
Ten minutes or so before rice is done cooking, place milk in a medium saucepan, add pepper and cornstarch. Whisk until the cornstarch doesn't stick to the bottom of the pan. Add in butter and turn a burner on to med-hi.
Continue whisking, or the milk will burn. Once the milk mix gets hot enough to start boiling, it will thicken, turn off burner, and whisk for another 30 seconds.
Add milk mixture to the tuna mixture. Stir mix until everything is well incorporated.
Add rice to tuna mixture. After everything is well incorporated, pour mix into a 9x13" casserole dish. Place in oven and bake for about 35 minutes. If you want a crispier top, raise temperature to 400°F.