Pulse the bread in a food processor to make coarse bread crumbs; transfer to a large bowl.
Add the crab, mayonnaise, green onions, lemon zest, Worcestershire, seafood seasoning, hot sauce, 1/4 teaspoon salt and a few grinds of pepper.
Mix gently to combine. Add the egg and mix gently but thoroughly.
Melt 3 tablespoons of the butter in a large nonstick skillet over medium heat.
Cook the crab cakes, in batches if necessary, turning once, until golden brown on both sides and heated through, about 4 minutes per side.
Transfer to a large plate and cover loosely with foil.
Melt the remaining 2 tablespoons butter in a large pot over medium heat.
Add the garlic and cook, stirring, until fragrant, about 2 minutes.
Add the spinach, in batches if necessary, and cook, tossing, until just wilted.
Season with salt and pepper.
Divide the spinach among four plates and top with the crab cakes.
Serve with a wedge of lemon to squeeze on top. Garnish with a dollop of sour cream, cocktail sauce, or a remolaude.