In a 12 to 14-inch saute pan, heat olive oil until smoking. Add garlic and rhubarb and cook until softened, about 4 to 5 minutes. Add littleneck clams and wine and bring to a boil. Cover and cook until all the clams are open, about 6 to 8 minutes.
Meanwhile, assemble crostini by first mixing roasted peppers, olive paste and remaining oil together. Divide pepper mixture among 12 toasts and place three on the rims of each of 4 plates. Add cockles to littleneck pan and cover. Cook until cockles open, about 1 minute. Add vinegar and escarole and season with salt and pepper. Stir through then divide escarole, clams and cockles among plates and serve.