Heat a grill pan over high heat; brush with vegetable oil. Grill 3 thick slices each fresh pineapple and white onion until charred, 1 to 2 minutes per side. Let cool, then chop. Halve, pit and dice 3 ripe avocados. Combine in a bowl with 1 minced habanero chile (seeded for less heat), 1/2 cup chopped cilantro, the juice of 2 limes and 1 teaspoon kosher salt. Mash a few times until combined. Fold in the grilled onion and pineapple; season with more salt.