Preheat oven to 425 degrees. Place the green beans and garlic in a 13 x 9-inch casserole dish, season with salt and pepper and toss with a few glugs of olive oil. You want to make sure the beans are nicely coated in the oil. Lay the cod over the beans and season with salt, pepper and then drizzle a bit of olive oil over the fish. Nestle the lemon halves in the corner of the dish, place the tomatoes evenly over the fish and beans, then sprinkle the walnuts over everything. Drizzle a little more olive oil over the dish, and cook for 15-20 minutes.