Heat a large skillet over medium heat. Drop in the 1 pound of lean ground beef and finely diced sweet yellow onion. Cook for 5 to 6 minutes, using a firm spatula to break the meat apart into uniform, fine crumbles until it is fully browned and no pink remains.

Tilt the skillet to thoroughly drain away any heavy pooling grease, then return the pan to the stove burner.

Sprinkle the taco seasoning and a splash of water directly over the beef, stirring continuously for 2 minutes over medium-low heat until the spices hydrate and glaze the meat bits. Pull the pan off the heat source.

Stir the shredded cheddar cheese, processed nacho cheese sauce, drained diced tomatoes, and chopped jalapeños straight into the warm skillet. Fold the mixture together until the residual heat melts the cheddar into a thick, uniform, and gooey beef matrix. Let the filling cool slightly for 5 minutes.

Lay an egg roll wrapper flat on a clean cutting board in a diamond position (with one corner pointing straight at you). Brush the outer edges of the wrapper lightly with the egg wash.

Spoon exactly 2 tablespoons of the cheesy beef filling into the center of the diamond. Lift the bottom corner pointing toward you and fold it firmly over the filling, tucking it snugly underneath.

Fold the left and right side corners inward toward the center, forming a shape resembling an open envelope. Roll the wrapper upward tightly away from you to seal the final corner completely. Repeat with the remaining wrappers, keeping the rolled egg rolls covered under a damp paper towel to prevent the skins from drying out.

Fill a deep, heavy-bottomed pot or Dutch oven with at least 2 inches of vegetable oil. Heat over medium-high heat until a candy thermometer registers exactly 350°F.

Working in small batches to avoid crowding the pan and dropping the oil temperature, slide the egg rolls carefully into the hot oil. Fry for 2 to 3 minutes per side, turning occasionally, until the wrappers bubble up and take on a deep, shatter-crisp golden-brown color.

Use a slotted spoon to lift the hot rolls out of the oil and transfer them to a wire cooling rack set over a baking sheet to drain. Placing them on a wire rack allows steam to escape from all sides, keeping the bottom skins from turning soggy. Let them cool for 3 minutes, then serve hot alongside sour cream and salsa.