Preheat oven to 425°F. Lay tortillas on a baking sheet in a single layer (do not overlap). Using 1 teaspoon of the oil and a pastry brush, brush both sides of each tortilla. Bake 8-10 minutes, flipping halfway through cooking time.*
Increase oven to 450°F. Clean the mushrooms and pat them dry. Trim dry ends then cut into quarters. In a large bowl, whisk together remaining 2 Tablespoons oil, cumin, chili powder, salt, and pepper. Add quartered mushrooms. Using a spatula, toss to evenly coat with oil mixture.
Spread spiced mushrooms on a baking sheet in a single layer. Do not overcrowd the pan. Roast, tossing halfway through cook time, until the mushrooms are easily pierced with a fork and caramelized, ~18-20 minutes.
While mushrooms are cooking, prepare guacamole. Cut avocados in half, remove pit, and use a spoon to scoop avocado out of the peel. Place in a medium bowl and mash with a fork or masher. Add onion, cilantro, garlic, and jalapeño to the bowl. Roll and press the whole lime on the counter or table to release juices. Cut in half and squeeze each half into the bowl. Stir to combine. Season to taste with salt.
Build tostadas by spreading a layer of guacamole on each shell. Top with mushrooms and additional toppings to taste. Enjoy!