Aioli: Drizzle the lemon juice over the saffron in a medium bowl. Set aside. Combine the garlic, salt, and egg yolks in another bowl. Whisk in the lemon and saffron mixture and season with freshly cracked black pepper, to taste. Slowly whisk in the extra-virgin olive oil, about 1 cup, leaving it a bit runny. Taste and with salt and freshly cracked black pepper, if needed.
Sandwich: Put a griddle over medium-high heat. Spread softened butter onto 1 side of each slice of bread. Put the bread onto the preheated griddle and toast on both sides, about 2 minutes.
Put a cast iron pan over medium-high heat and add about 1 cup of blended oil, to shallow fry.
Season the rice flour with salt and dredge the soft-shell crabs in the flour mixture. Once the oil is heated, carefully add the crabs and fry, about 2 minutes per side. Top 4 of the toasted bread slices with tomato slices, sea salt, pile of peppercress, cooked bacon slices, 3 soft-shell crabs, lemon zest, and drizzle with lemon saffron aioli.
Top with remaining bread and slice in half.