In a large skillet, heat 2 teaspoons of olive oil over medium heat. Add the sliced onion along with a pinch of salt, and sauté until the onions become translucent and slightly golden, about 3-4 minutes.

Once the onions are ready, add 1-2 cloves of diced garlic to the skillet. Continue to sauté for another 1-2 minutes until the garlic becomes fragrant.

Incorporate the sliced Portobello and Swiss brown mushrooms into the skillet. Drizzle with an additional teaspoon of olive oil, and stir-fry for about 5-7 minutes until golden brown and tender.

In a separate bowl, combine fresh arugula leaves with a drizzle of balsamic vinegar, a touch of olive oil, and a sprinkle of salt. Toss lightly.

To assemble, spread a layer of ricotta cheese on the toasted sourdough, layer with arugula salad, then pile on sautéed mushrooms. Sprinkle chili flakes if desired.