In a large saute pan over medium-high heat, add the olive oil. Add the onions, leeks, and salt and pepper, to taste. Cook stirring frequently until caramelized, about 15 minutes. Add the garlic and shiitake mushrooms. Continue cooking for an additional 3 to 4 minutes. Stir in the vegetable broth and bring to a simmer. Add the peas, red pepper flakes and scallions.
Season the broth with salt and pepper, to taste, and add a dash of sherry vinegar. Cook for 1 minute. Keep warm. Season the scallops with salt and pepper, to taste. In another large saute pan over medium-high heat, add the butter. Add the scallops and sear for 4 minutes. Flip the scallops, then squeeze in some lemon juice. Baste the scallops for 1 minute to finish cooking. Remove from heat. Spoon the caramelized onion mixture on serving plates and top each plate with 4 scallops. Serve.