To make the succotash, heat the oil in a large skillet. Add the onions, garlic, and both peppers and cooking until begging to soften, 3 to 4 minutes. Add the corn, stock, salt and pepper and bring to a boil. Reduce the heat to a simmer and cook for 10-15 minutes, until flavorful and the veggies are tender. Season to taste and cool. When cooled, stir in the edamame and tomatoes.
Pat the cod fillets dry with paper towels and season with salt and pepper on all sides.
Heat a large skillet over medium-high heat with 1 tablespoon oil. When hot, gently place the fillets into the skillet and cook for 5 minutes, undisturbed. Add the remaining tablespoon (or more if you like) of oil and use a spoon to base the fish with the hot oil until they turn opaque. Immediately remove from the heat to prevent over-cooking.
In a small bowl, combine all of the tartar sauce ingredients.
Divide the succotash between plates. Top with cod fillets and mizuna for garnish and serve with tartar sauce and lemon wedges on the side.