Prepare rice according to package directions.
While rice is cooking, sauté onion in 1-2 TBSP oil over medium-low heat until translucent (approx 5-7 minutes). This is a very gentle sauté, you don’t want to burn or brown the onion.Optional: Add a small dash of paprika and oregano here.
Open and drain tuna cans. Add the tuna to the skillet with the onions and reduce heat to low. Stir with a wooden spoon or spatula, breaking up larger chunks of tuna.
In a 4 cup liquid measuring cup combine milk, mayo, flour, salt, and a few grinds of black pepper. Whisk to combine. Add oregano and paprika and whisk again until smooth and no visible lumps of flour or mayo remain.
Return skillet to medium heat and pour in the milk mixture. Bring to a low simmer, stirring occasionally to prevent tuna from sticking to the bottom of the pan.
Reduce heat to low and simmer, covered, for 10 minutes, and then uncovered 10-15 minutes more, stirring occasionally, until thickened.
Serve in bowls over rice. Sprinkle with additional oregano to garnish.