Step 1

Position a rack in the middle of the oven and preheat to 400 degrees. Line a plate with a clean dish towel. Halve the squash lengthwise, scoop out and discard the seeds (or save for another use), and cut the unpeeled squash into 1/2-inch half-rings. In a large bowl, toss the squash with 1 tablespoon of the olive oil. Arrange on a large, rimmed baking sheet in a single layer, and roast for 10 minutes. Flip the pieces and continue roasting for about 5 minutes, or until the squash is cooked through and golden brown. Let the squash cool in the pan. Meanwhile, in a 4-quart saucepan over medium-high heat, heat the sunflower oil until it reaches 375 degrees on an instant-read thermometer. Fry the corn tortillas, one at a time, until deep golden brown and crisp, 1 to 2 minutes. Use tongs to keep each tortilla submerged as it fries, and flip it as needed. When the tortilla is ready, lift it out of the oil and gently shake to let as much excess oil drip off as possible. Drain on the towel-lined plate, and repeat with the remaining tortillas. In the same bowl you used for the squash, whisk together the remaining olive oil, lime juice, ground ancho, garlic powder, onion powder, salt, smoked paprika, cumin, allspice, cinnamon and cloves. Return the squash to the bowl and gently toss to coat the squash in the spice mixture. Smear 3 tablespoons of refried beans on each tostada. Divide the squash pieces among the tostadas and top with avocado slices, salsa and cilantro, if using. Serve warm.