Cook rice according to package directions. Make the tuna newberg sauce while the rice cooks.
Heat pan over medium heat. Add oil, then reduce heat to medium low. Sweat onions, stirring occasionally, until translucent (5-7 minutes). This is a very gentle process, you don’t want to burn or brown the onion.
Add the tuna to the skillet. Stir with a wooden spoon or spatula, breaking up larger chunks of tuna.
In a 4 cup liquid measuring cup combine milk, mayo, flour, paprika, oregano, salt, and black pepper. Whisk well to combine.
Return skillet to medium heat and pour in the milk mixture. Bring to a low simmer, stirring frequently to prevent tuna from sticking to the bottom of the pan.
Reduce heat to low and continue simmering, covered, for 15-20 minutes, and then uncovered 10-20 minutes more, stirring occasionally, until thickened. Adjust seasoning to taste.
Serve in bowls over rice. Sprinkle with additional oregano or paprika to garnish.
Leftovers: Reheat in a microwave or eat cold on rice cakes.
I've written this recipe with the ingredients and ingredient quantities that I always use, but I encourage you to think of them as guidelines more than rules. It's hard to mess this up, so don't sweat it if you don't have everything in just the right amounts.