Preheat the oven to 400 degrees . On a baking sheet, drizzle the cipollini onions with 1 tbsp. oil. Roast until tender and browned in spots, 30 minutes; let cool, then quarter.
Meanwhile, in a large skillet, stir the chiles over medium heat until lightly toasted, 1 to 2 minutes. Transfer to a medium saucepan. Add 2 cloves garlic and water just to cover. Bring to a simmer over medium and cook until chiles are tender, about 30 minutes. Strain, discarding liquid. Return the chile mixture to the saucepan.
Heat a griddle or large skillet over medium. Add the red onion and remaining 2 cloves garlic. Cook, turning occasionally, until softened and browned, about 8 minutes. Add the red onion and garlic, the pineapple slice, vinegar, paprika and 1/2 cup water to the saucepan with the chiles. Simmer, stirring often, until the onion and pineapple are very tender and the sauce thickens, 30 to 40 minutes. Transfer to a blender; puree. Season the al pastor sauce with salt and pepper.
In a large saucepan, heat 1/4 cup oil over medium-high until it begins to shimmer. Add the cauliflower and cook, stirring often, until golden brown, about 8 minutes. Add the white onion and Serrano chiles and cook, stirring often, until the onion softens, about 4 minutes. Reduce the heat to medium; add the al pastor sauce. Cook, stirring occasionally and adding water by the tablespoon if too dry, until the cauliflower is soft but not falling apart, about 15 minutes. Season with salt. Let stand at least 1 hour to let the cauliflower absorb the flavors.
Heat the remaining 1/4 cup oil in a large skillet. Add the cauliflower. Cook, stirring occasionally, until cauliflower begins to char in spots, about 5 minutes. Add the pineapple cubes and cipollini onions; stir until heated through, about 1 minute. Season with salt; stir in the cilantro. Serve the cauliflower with the tortillas.