Preheat the oven to 180º C (350 º F)
Toss the eggs, the flour, the milk, the cheese and the oil into the blender and blend in high speed for 5 minutes.
Add the salt and the baking powder and blend in low speed until you get a homogeneous dough.
Now to make the filling we need to heat a big pan and pour a little bit of olive oil and saute the onions, the pepper, and the garlic for a couple minutes. Then add the tuna and cook over medium heat for five minutes.
To assemble the tuna tart: first, you will need to grease a tart pan. Then pour half of the dough. Above place a layer of the tuna filling and cover with the remaining dough. Take it to the oven for 25 minutes.
Let the tuna tart chill completely. Then cut it into cubes and you can serve or store up to 4 days.