Step 1

In a large skillet over medium-high heat, heat the olive oil. Add the mushrooms and onion and sauté, stirring frequently, for about 4 minutes.

Step 2

Reduce the heat to low, add the garlic, and sauté, stirring continuously, for about 2 minutes. Add the scallion and a pinch each salt and pepper and stir for 1 minute. The sautéed ingredients should be tender but a little crunchy with a tinge of a light brown color. Transfer to a bowl.

Step 3

Return the skillet to medium-high heat. Butter 1 side of each slice sourdough bread and place the bread in the skillet, buttered side down. Divvy the cheese between both slices bread. Working in a zigzag motion, squeeze the Sriracha sauce over the cheese.

Step 4

Add the sautéed vegetables, tomato, and fresh basil, if using, and avocado to a slice of cheese-topped bread. Top with the other slice bread, buttered side up. Cook, flipping once, until crisp and golden brown, about 3 minutes per side, during which time you may want to reduce the heat to medium so the cheese melts before the bread browns too much.

Step 5

Serve the veggie grilled cheese sandwich immediately.