For the salsa:

Heat the broiler to high and arrange a rack in the upper third of the oven.
Place the tomato halves skin-side up on a baking sheet.
Scatter the onion, garlic, and Serrano around the tomatoes.
Broil until the tomato skins start to blacken and blister, about 7 minutes.
Transfer the vegetables to a blender, add the measured lime juice and salt, and blend into a smooth purée.
Taste and season with additional salt and lime juice as needed.
Transfer to a small saucepan and keep warm over low heat.

For the chilies:

Blister the chilies to nearly black by either placing over open flame and turning
- or -
Place in broiler and broil until the skins start to make popping sounds and char and turn black in places, take the chilies out and flip them over.

Insert a paring knife in the top of the chili near outside edge and slice around to remove core and seeds. Scrape inside with a small spoon to remove any remaining seeds.

Place in a sealed plastic bag to steam for about 10 minutes.
Once the skin comes off easily, peel each chili.

Insert a paring knife in the top off center to cut around stem and core.
Remove stem and seeds, scraping inside with a small spoon to remove any remaining ribs and seeds.

Fill each cored pepper with cheese.

Whip the egg whites with a dash of cream of tartar ( if using ) at high speed with an electric mixer, until stiff peaks have formed.

Beat the egg yolks with one tablespoon flour and salt. Spoon about 1/4 cup of beaten egg whites into yolk mixture to lighten. Fold yolk mixture into whites until uniform color.

Heat the oil in a skillet until a drop of water sizzles when dropped into the pan.

Dip chili into egg batter and place in pan. Fry till golden ( about 2 minutes ) then flip to brown other side.
Place on paper towels to drain.

Repeat for remaining chilies.

Place one or two Relenos on each plate and pour warm salsa over them. Serve.