For the salsa roja, preheat the grill to high. Make sure an oven rack is in the second highest position. Line a standard baking sheet with aluminum foil or bake directly on the baking sheet.
Place the tomatoes, garlic and onion on the prepared baking sheet in a single layer. Season with salt and pepper and coat with oil. Grill for 10 minutes, or until you get blisters and the vegetables are soft. Return the vegetables to the baking sheet and continue grilling for an additional 3-5 minutes, until cooked through and nicely browned on the other side. If using the California chili, add it to the pan at the last minute to lightly toast it, being careful not to burn it.
Remove everything from the oven. If you added the California chili, fill a bowl with 2 cups of boiling water and add the freshly roasted chili to submerge. Leave to soften for about 5 minutes before blending.
Remove the chili from the water (discard the water) and add the chili and vegetables to the baking sheet in a wide-mouthed storage jar, discarding any juices from the bottom of the baking sheet. Add the lime and cilantro to the jar and blend with an immersion blender until the mixture is almost smooth. Taste and season generously with more salt and pepper as needed. Leave to cool, then cover and store in the refrigerator.
For the flautas, preheat the oven to 450°F. Grease a 9 x 13 inch baking dish with oil.
Season the shredded chicken with a little salt and pepper. Heat a large, heavy-bottomed skillet (ideally cast iron) over high heat, without adding oil. Place 1 tortilla in the pan to warm it for about a minute on each side. This helps make the tortillas soft for rolling. Transfer the tortilla to a plate and put the next tortilla in the pan.
Meanwhile, fill the first warmed tortilla with about ¼ cup seasoned chicken. Roll the tortilla like a tight cigar and place, seam side down, in the prepared baking dish. Continue with the remaining tortillas.
Generously spray the tops of the flautas with oil and bake for about 15 minutes, until crispy and golden.
While the flautas are cooking, prepare the guacamole. In a small bowl, mash the avocados and stir in the lime juice. Season with salt and pepper to taste.
Serve the flautas with the salsa, guacamole and hot sauce.