Empanada with tuna

Oil, extra-virgin olive	1/3 cup +
Onion, white, quartered lengthwise, thinly sliced crosswise	1 large
Bell peppers, green, stemmed, seeded, thinly sliced	2 medium
Garlic, thinly sliced	8 cloves
Salt, kosher	½ teaspoon +
Pepper, freshly ground
Saffron, finely ground	Large pinch
Water, boiling	¼ cup + 1 tablespoon
Tomatoes, halved crosswise	2 medium
Piquillo peppers, drained, thinly sliced	3 to 5
Paprika, sweet	1 teaspoon
Tuna, preferably ventresca (from the belly), drained, well flaked	14 ounces
Olives, Spanish, pimento stuffed, sliced	½ cup
Parsley, flat-leaf, finely chopped	½ cup
Lemon juice	1 tablespoon
Flour, all-purpose	3 cups
Butter, cold, cut into cubes	¾ cup
Milk, cold	¼ cup + ¼ cup
Eggs	1 large + 1 large

1. In a large skillet, heat 1/3-cup oil until shimmering. 
2. Add the onion, and cook over moderate heat, stirring occasionally, until just starting to soften, about 5 minutes. 
3. Add the bell peppers, garlic, and a generous pinch of salt; cover, and cook over moderately low heat, stirring occasionally, until very soft, about 25 minutes: add a bit of water if the pan is too dry. 
4. In a small heatproof bowl, mix the saffron with the boiling water until dissolved; let stand until cooled, about 10 minutes. 
5. Grate the tomatoes on the large holes of a box grater set in a bowl until only the skins remain; discard the skins. 
6. Add the piquillo peppers, tomatoes, steeped saffron, and the paprika to the skillet, and cook over moderate heat, stirring occasionally, until the mixture is very thick, about 10 minutes. 
7. Scrape the mixture into a colander, and let drain for 10 minutes. 
8. In a large bowl, combine the pepper mixture with the tuna, olives, parsley, and lemon juice; season with salt and pepper. 
9. Let the filling cool completely, about 45 minutes. 
10. Preheat the oven to 375°F. 
11. In a food processor, pulse together the flour and salt. 
12. Add the butter, and pulse until crumbly. 
13. Add ¼-milk and the egg, and pulse until the mixture starts to look wet and clump together. 
14. Gradually pulse in the remaining milk as needed until dough forms. 
15. Turn out on a lightly floured surface, and kneed six or seven times to make the dough more elastic. 
16. Divide into twelve to sixteen dough balls, and roll into 8- to 10-inch rounds. 
17. Place a heaping spoonful in the center of a round (for 16 empanadas, I measured 54 grams per empanada), fold the dough over, and crimp seal the edges; transfer to a baking sheet, and repeat with the remaining dough balls.
18. In a small bowl, whisk an egg with 1-tablespoon water, and then brush each of the empanadas.
19. Bake until golden brown, about 30 minutes. 
20. Let cool for about 10 minutes, and devour!

Recipe adapted from Food&Wine Magazine, September 2017