For the provencal sauce: In a large saucepot, heat the grapeseed oil over medium-low heat. Add the tomatoes, garlic and onions and sweat until slightly softened, about 3 minutes. Deglaze with the white wine and simmer briefly. Add the chicken stock, tomato juice and tomato paste and cook an additional 3 minutes. Season with a pinch of kosher salt and black pepper. Reduce the heat to a low simmer and cook until the sauce slightly thickens.
For the frog legs: Mix together the flour, 2 tablespoons salt and 1 tablespoon pepper in a shallow bowl. Toss the legs in the seasoned flour and coat completely.
Heat a saute pan over medium heat until hot. Add the grapeseed oil to the pan and saute the frog legs until golden brown, 4 to 5 minutes. Transfer the legs to the sauce and braise until the legs are cooked through and the sauce is slightly thickened, 4 to 5 minutes. Swirl in the butter, chopped parsley and lemon juice.
Place some steamed rice in the center of each plate. Spoon some frog legs and sauce over the rice. Add additional parsley and lemon wedges for garnish if desired.