This is a holiday favorite that I make every year. It's always a big hit with family and friends.
1. Make the tomalley sauce by pushing the tomalley through a sieve with a wooden spoon into a small pot. Add the chili flakes, thyme, garlic, and lemon juice. Pour in the reserved crab juices, mix, and bring to a quick simmer. Give it a final quick stir and remove the pan from the heat. Pour the tomalley mixture into a small mixing bowl and allow it to cool. (Start roasting the crab once the tomalley sauce is cooling.) As soon as the tomalley sauce feels lukewarm to the touch, add mayonnaise or aioli and taste it to check seasonings. Add more salt, pepper, or thyme if you like.
3. Place a rack in the oven in the middle or lower third and preheat the oven to 500°F.
4. On the stove, in a large oven-proof sauté pan or casserole, heat the butter, olive oil, garlic, thyme, and lemon zest over medium-high heat until hot. Add the pieces of crab, season with salt, pepper, and the remaining thyme and mix well. Place the pan in the oven and cook until the crab is hot throughout and the garlic is golden brown. (Watch carefully; the crab heats very quickly and at 500° the garlic can burn easily.) Drizzle with 4 tablespoons of lemon juice and scatter the parsley over the top.
5. To serve, stack the crab on a platter and drizzle the tomalley sauce over the top or serve the tomalley sauce in a small bowl and allow your guests to help themselves. Garnish the platter with lemon wedges and slices of garlic-rubbed toast. Serve immediately.