Heat the oven to 375°F. Put the tomatoes in a baking dish and roast until they start to char and shrivel, 40 to 50 minutes.
Put the oil in a large pot over medium heat. When it’s hot, add the onion and garlic and sprinkle with salt and pepper. Cook, stirring occasionally, until soft and starting to brown, 5 to 8 minutes. Add the cumin and oregano and cook, stirring constantly, until fragrant, about 1 minute. Add the roasted tomatoes, the chipotle, adobo, and stock and sprinkle with salt and pepper. Bring the mixture to a boil, then adjust the heat so it bubbles gently. Cook, stirring occasionally, until the tomatoes have broken down and the sauce has thickened a little, 25 to 30 minutes. Use an immersion blender to puree the mixture directly in the pot, or carefully transfer the mixture in batches to a blender to puree. Taste and adjust the seasoning.
Cut the core from the cauliflower, then roughly chop the florets into pieces about 1/2 inch in size. Add the cauliflower to the sauce and bring the mixture to a boil, then adjust the heat so the mixture bubbles gently. Cook, uncovered and stirring once in a while, until the cauliflower is tender, 10 to 15 minutes. Taste, adjust the seasoning, and keep warm.
Pour about 1/2 inch oil into a large skillet over medium-high heat. Put a wire rack on a large baking sheet. When it’s hot, put a tortilla in the skillet and let it cook until it starts blistering but is still soft, less than 15 seconds. Carefully turn the tortilla with tongs, and immediately fold it over and hold it for a few seconds to form a taco shell. Once the tortilla holds its shape, turn it every few seconds, until it’s crisp and golden all over, 15 to 30 seconds more. Transfer to the wire rack to drain. Repeat with the remaining tortillas, adjusting the heat and adding more oil as necessary.
To assemble the tacos, spoon the cauliflower into the crisp shells and top with radish slices, avocado, and mint.