Add sliced onions to a heatproof bowl and cover briefly with boiling water, then drain. Combine onions with salt and lime juice and let pickle.
Bake bacon at 400°F until crisp, then drain on paper towels.
Warm a skillet or griddle. Spread chili-garlic butter on one side of each bread slice. Place half the slices butter-side down on the skillet. Layer each with cheese, bacon, and pickled onions. Top with remaining slices butter-side up.
Cook until the cheese melts and the bread browns, flipping halfway.
Rest briefly, then slice and serve with chipotle crema.