In a food processor, combine fresh basil leaves, cashews, garlic cloves, nutritional yeast, and lemon juice. Process until finely chopped, then drizzle in olive oil until creamy. Season with salt and pepper to taste, then set aside.

Heat a splash of olive oil in a skillet over medium heat. Add sliced mushrooms and sauté for 5-7 minutes until tender and golden brown, seasoned with salt and pepper. Remove from heat.

Toast slices of whole-grain bread until golden and crispy to add crunch to the sandwich.

Spread homemade vegan pesto on each toasted bread slice, then layer with sautéed mushrooms, avocado, tomato, red onion, and spinach. Top with remaining bread, pesto side down.

Cut sandwiches in half and serve immediately while warm and crispy.