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Preheat broiler. Spray a large baking sheet with oil or cooking spray.
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Heat the avocado oil in a large skillet over medium-high heat. Stir in the fajita seasoning, then add the sliced onion. Cook for about 5 minutes, or until softened, then stir in the zucchini and corn. Cook until zucchini is softened, about 3 minutes. Fold in the beans and remove from heat.
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Arrange the tortillas on the prepared baking sheet. Broil until crisp, about 2 minutes - broilers can vary wildly, so keep a close eye on the tortillas to make sure they don't burn.
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Divide the cheese and fajita vegetables onto each tortilla. Return to the oven and broil until the cheese has melted, about 2 minutes.
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Transfer the tostadas to plates and add toppings.
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To make the fajita seasoning: Combine all of the ingredients in a small jar, cover, and shake to combine.