Preheat the oven to 375°F.Remove the stems from the portobello caps, finely chop them and place in a mixing bowl. Remove the mushroom gills (the paper like fins underneath the mushroom cap), then brush the caps clean with a damp kitchen towel.Arrange the caps in a single layer in a shallow baking pan that's been coated with nonstick cooking spray. Season each mushroom cap with salt and pepper and set aside.Add the crab, spinach, egg, panko, onion, red pepper, garlic, thyme, lemon juice, salt and pepper to the chopped portobello stems. Gently combine all the ingredients and divide the stuffing between the mushroom caps. Gently compress the stuffing to form a slight mound in each mushroom.Bake until the crab mixture is set and light golden brown, which will take approximately 8-10 minutes. Remove from the oven, top each mushroom with grated cheese and bake for an additional 3-5 minutes, or until the cheese is melted and bubbly.Serve with lemon wedges and hot sauce if desired.