Preheat a large griddle, cast-iron pan, or outdoor grill over high heat until very hot.

While the surface warms, remove husks from the tomatillos and rinse away any sticky residue. Peel and halve the onion and rinse the tomatoes and serrano pepper.

Brush a thin layer of vegetable oil over the hot griddle or pan to prevent sticking.

Place the tomatoes, tomatillos, onion halves, and serrano on the hot surface. Cook about 5 minutes, then flip each piece to the opposite side.

Continue turning the vegetables every few minutes until they are charred on multiple sides and softened—this may take up to 30 minutes. Expect sizzling and occasional popping.

Remove the roasted vegetables from heat and allow them to cool for about 15 minutes.

Trim any stems from the tomatoes, tomatillos, and serrano before processing.

Place the cooled, charred vegetables into a blender along with the lime (or lime juice), cilantro, and kosher salt.

Pulse in short bursts on medium speed until the mixture reaches a medium-textured consistency—slightly chunky but without large pieces. Adjust the number of pulses to make the salsa thicker or smoother.

Taste and add additional salt if needed; tomatoes and tomatillos can absorb salt so adjust to preference.

Transfer the salsa to a container and refrigerate for about 2 hours if you prefer it chilled, or serve immediately at room temperature.

Serve with tortilla chips or alongside your favorite savory dishes.