In a large braiser over medium heat melt butter; add onions, garlic, scallion whites, thyme, and bay leaves. Allow to sweat for 5 minutes then add fish stock, raise heat and bring to a simmer. Add potatoes, squash, chayote, Scotch Bonnet pepper, and a little salt. When potatoes are tender, begin layering fish, start with tile, then bass, snapper, and shrimp then adjust with salt and pepper. Simmer fish until firm and flakes to the touch, about 10 to15 minutes. Sprinkle scallion greens and cilantro over top and serve.