Bring a large pot of salted water to a boil. Cook penne according to package directions until al dente. Before draining, reserve 1 cup of pasta water, then drain but do not rinse.

While the pasta cooks, heat olive oil in a large skillet over medium-high heat. Add sliced mushrooms and cook, stirring occasionally, for 5–7 minutes until golden and released moisture has evaporated.

Reduce heat to medium. Add minced garlic to the skillet and cook for 30 seconds until fragrant. Add baby spinach in handfuls, stirring until just wilted, about 1–2 minutes. Remove from heat.

Drain the soaked cashews and add them to a blender along with vegetable broth, nutritional yeast, lemon juice, salt, and black pepper. Blend on high until silky smooth, scraping down sides as needed.

Pour the cashew cream into the skillet with the mushroom-spinach mixture. Add the drained pasta and toss well. Thin the sauce with reserved pasta water, 2 tablespoons at a time, until it reaches a velvety consistency that coats every noodle.

Divide among bowls. Sprinkle with optional red pepper flakes, fresh parsley, and an extra dusting of nutritional yeast. Serve immediately.