Step 1In a sauté pan, heat the olive oil over medium heat. Add the onion and sweat, stirring occasionally, until the onion is softened and translucent, 6 to 8 minutes.
Step 2Stir in the piquillo peppers, along with the water. Increase the heat to medium high and bring to a gentle simmer.
Step 3 Taste the sauce, adding one-half teaspoon of sugar, or as desired, to sweeten slightly, along with three-fourths teaspoon salt, or to taste.
Step 4Remove the sauce to a blender and purée until smooth; strain the sauce if desired. Return the sauce to the sauté pan and gently simmer for 10 minutes to marry the flavors. Thin if desired with additional water, and taste and adjust the sugar and salt again as needed. This makes about 2 cups sauce.
Step 1Make the bechamel sauce: In a medium saucepan, heat the oil over medium heat. Whisk in the flour to form a roux. Slowly add the milk, whisking to make sure the flour does not clump. Bring the mixture to a gentle simmer, whisking frequently. Cook uncovered for 20 minutes, stirring frequently (be sure to scrape the bottom of the pan to prevent the flour from settling and burning). Taste the mixture, seasoning with salt and adding one-eighth teaspoon ground nutmeg, or as desired, for flavor. Strain the sauce, discarding any solids. This makes about 11/4 cups bechamel; the bechamel will continue to thicken as it cools while the meat is cooked.
Step 2In a sauté pan, heat the olive oil over medium-high heat. Add the onion and cook until translucent, 4 to 6 minutes, then stir in the garlic and cook until aromatic, about 1 minute. Stir in the ground pork and beef, and cook until browned, about 10 minutes. Stir in the parsley, then taste and season with 11/4 teaspoons salt and one-fourth teaspoon pepper, or as desired. Remove from heat and drain the meat, discarding the fat. Set aside to cool.
Step 3In a large bowl, combine the meat with one-half cup bechamel and the bread crumbs, stirring or kneading to form a cohesive filling (if the filling is too crumbly, add bechamel as needed). This makes a generous 3 cups filling.
Step 4 Prepare to deep-fry: Fill a medium, heavy-bottomed stock pot with enough oil to come up the sides by at least 3 inches, and heat the oil to 350 degrees.
Step 5Stuff each pepper with 1 to 2 tablespoons filling, depending on the size of each pepper. Dip the stuffed peppers in the egg batter and fry until the crust is a light golden brown. Remove and drain the peppers on paper towels, then serve the hot peppers, garnished with chopped parsley and with the sauce on the side.