Galician Tuna Empanada with Melting Onions (Empanada gallega de atún)

From The New Spanish Table by Anya von Bremzen.
Ingredients:
1/3 cup extra-virgin olive oil, plus more if needed
2 medium-size garlic cloves, minced
2 large white onions, quartered and thinly sliced
3 large red bell peppers, cored, seeded, and thinly sliced
1 medium-size green bell pepper, cored, seeded, and thinly sliced
1 large ripe tomato, cut in half and grated on a box grater, skin discarded
2 tsp. sweet (not smoked) paprika
1 small pinch of saffron threads, pulverized in a mortar and steeped in 2 Tbsp. very hot water
18 oz. imported solid oil-packed tuna, or 3 cans (each 6 oz.) tuna in olive oil, drained and flaked thoroughly in with a fork
3 Tbsp. minced fresh flat-leaf parsley
Coarse salt
Freshly ground black pepper
3/4 c. pitted manzanilla olives, thinly sliced
All-purpose flour for dusting the work surface
Olive oil and saffron pastry
1 large egg yolk
1 1/2 tsp. milk

Place the olive oil in a large skillet over medium heat. Add the garlic and cook for 1 minute. Add the onions and cook until translucent, about 5 minutes. Add the red and green peppers and cook until they begin to soften, 7 to 8 minutes, adding a little more olive oil if the skillet looks dry. Reduce the heat to low and cook, stirring occasionally, until the vegetables are very soft but not brown, about 12 minutes. Add the tomato, paprika, and saffron, cover the skillet, and simmer over very low heat, stirring occasionally, until the vegetable are reduced to a jamlike consistency, about 15 minutes. Let the vegetable mixture cool, then stir in the tuna and parsley. Season with salt and black pepper to taste. The empanada filling can be prepared up to a day ahead and refrigerated, covered.
Place an oven rack in the center of the oven and preheat the oven to 375°F. Lightly brush a 17- by 11-inch baking sheet with olive oil.
Lightly flour a work surface. Using a floured rolling pin, roll out the larger pastry ball to a roughly 19- by 12-inch rectangle. Transfer it to the oiled baking sheet; it will overhang the edge slightly. Roll out the remaining pastry to a rectangle that is slightly smaller than the first. Spread the filling evenly over the pastry on the baking sheet. Scatter the olives evenly on top. Cover the filling with the second pastry rectangle. Fold the edges of the bottom crust up over the top and crimp them decoratively.
Place the egg yolk and milk in a small bowl and whisk to mix. Brush the top of the empanada with this egg wash. Using a sharp knife, make several slits all over the top crust to allow steam to escape.
Bake the empanada on the center rack until golden brown and baked through, about 45 minutes. Cover the empanada loosely with a clean kitchen towel and let cool until slightly warm or room temperature before serving. Makes 1 large empanada; serves 12 to 14 as a tapa, 8 as a light main course.