How to Make It
In a medium bowl, combine the mayonnaise with the yogurt, buttermilk and lemon juice. Whisk in the mashed garlic, mustard seeds, cracked black peppercorns and dill seeds until combined. Whisk in the grapeseed oil. Add the sliced red onion, radish wedges and cucumber slices and season well with salt. Cover the salad and refrigerate.
Put the flour, eggs and panko in 3 shallow bowls and season each with salt and ground black pepper. Lightly beat the eggs. Pat the oysters dry with paper towels. Lightly dredge them in the flour, dip them in the eggs and then in the panko, pressing lightly to help the crumbs adhere. Transfer the breaded oysters to a baking sheet lined with wax paper.
In a large skillet, heat the canola oil until shimmering. Working in batches, fry the oysters over high heat, turning once, until the coating is crisp and golden, 2 to 3 minutes. Transfer to paper towels to drain. Season the oysters with salt.
Arrange the fried oysters on a platter and spoon the creamy radish and cucumber salad on the side. Garnish with the watercress and serve.