Rinse hibiscus very well to make sure all dust is removed. Place clean hibiscus in a pot over medium heat, cover with water, and boil for 10 minutes. Remove hibiscus from heat and allow to steep for at least 2 hours, until hibiscus is very soft. Drain well.
Heat oil in a large skillet over medium heat and cook 1 onion and 2 cloves garlic until soft and translucent, about 2 minutes. Stir in drained hibiscus and season with salt and pepper. Cook for 5 minutes, stirring constantly, until hibiscus turns a deep red color.
Combine tomatillos, avocado, 1/4 onion, serrano peppers, 1 clove garlic, and 2 tablespoons cilantro; blend until smooth. Season salsa verde with salt.
Warm corn tortillas in a skillet. Divide hibiscus mixture amongst tortillas and top with pineapple, onion, and cilantro. Serve with salsa verde and lime.