In a large bowl, combine the buttermilk and 2 tablespoons of mayonnaise; season with salt and black pepper. Add the livers. In a large, resealable plastic bag, mix the flour, onion powder and cayenne with 1 teaspoon each of salt and pepper. Drain the livers and add them to the seasoned flour; toss to coat.
In each of 2 large skillets, heat 1/4 inch of oil until shimmering. Add the chicken livers to the pans and fry over moderately high heat for 6 to 8 minutes, turning once, until browned and crisp. (Be carefulthe livers splatter while they fry.) Drain the chicken livers on paper towels; sprinkle with salt.
Pour off all but 2 tablespoons of the oil in one of the skillets. Add the onions; cook over moderate heat until softened, 8 minutes.
Spread mayonnaise on the cut sides of the baguettes and fill with the chicken livers, sautéed onions and arugula. Squeeze the lemon over the chicken livers, close the baguettes and cut them into quarters. Serve.