Lay 1 chile on a work surface so that it sits flat naturally, without rolling.
Using a paring knife, make two cuts forming a “T” by first slicing down the middle of the chile lengthwise from stem to tip, then making a second cut perpendicular to the first about a 1/2 inch from the stem, slicing only halfway through the chile (be careful not to cut off the stem end completely).
Carefully open the flaps to expose the interior of the chile and, using the paring knife, carefully cut out and remove the core.
Scrape the inside with a small spoon to remove the seeds, ribs, and any remaining core.
Repeat with the remaining peppers.
If you are working with a gas stove - turn 2 gas burners to medium-high heat.
Place 1 chile directly on each burner and roast, turning occasionally with tongs, until blackened and blistered on all sides, about 5 to 7 minutes.
If you have an electric stove - heat the broiler to high and arrange a rack in the upper third of the oven.
Place all of the chilies directly on the rack.
Broil, turning occasionally with tongs, until the chilies blacken and blister on all sides, about 8 to 10 minutes.
Remove and put in a sealed container. Cover tightly and let the chilies steam until cool enough to handle, about 15 minutes.
The chilies will be softer using the broiler rather than a direct flame, so be careful not to tear them while stuffing.
Using a butter knife, scrape away and discard the chile skins, being careful not to tear the chilies; set the chilies aside.
Heat the oven to 250°F and arrange a rack in the middle.
Place a wire rack over a baking sheet and set aside.
Season the inside and outside of the chilies with salt and pepper.
Stuff each chile, being careful not to tear them, with a quarter of the cheese or vegetables and close the flaps over the cheese; set the chilies aside.
Separate the eggs whites from the yolks while they’re still cold, which will make them easier to handle, then let them come to room temperature. Make sure there are no traces of yolk, or the whites will not whip properly.
Whisk the egg yolks in a medium bowl until lightened in color and frothy, about 2 minutes; set aside.
Place the egg whites and salt in a clean, dry bowl of a stand mixer fitted with a whisk attachment.
Beat on high speed until stiff peaks form, about 1 to 2 minutes.
Remove the bowl from the mixer, add the egg yolks, and fold with a rubber spatula until just combined (do not deflate the egg whites); set aside.
Heat the oil in a large frying pan over medium-high heat until hot, about 4 minutes.
Working with 1 chile at a time, drop 1/2 cup of the egg batter into the oil and use a rubber spatula to spread it to about the same size as the stuffed chile.
Lay the chile seam-side down on top of the mound of batter.
Drop another 1/2 cup of batter on top of the chile, spreading it with the rubber spatula to cover the sides and encase the chile.
Cook without disturbing until the bottom of the chiles rellenos is golden brown, about 2 to 3 minutes.
Using a flat spatula and a fork, carefully flip the chiles rellenos over and cook until the other side is golden brown, about 2 to 3 minutes. (If the sides of the chile are not brown, use a spatula or tongs to turn it onto each side to brown.)
Transfer the chiles rellenos to the rack set over the baking sheet, season with salt, and place in the oven to keep warm.
Repeat with the remaining stuffed chilies.
To serve, spread 1/3 cup of the roasted tomato salsa on each plate and top with a chiles rellenos.
Serve immediately, placing the remaining salsa on the side.