Cover the bottom of a large skillet with olive oil and heat over medium-high heat. When hot add the onion, corn, sweet pepper, and mushrooms; sauté for 5 minutes, or until the vegetables are tender.
Slowly pour milk over the corn and season with salt and pepper. Reduce heat to medium and cook for 8 minutes, or until all the liquid has evaporated, stirring occasionally. Add heavy cream. Cover and let simmer on low until ready to use.
Meanwhile, place butter in a medium skillet. Melt the butter over medium heat. Add garlic, basil, and lemon juice.
Cut the fish into 4 fillets and add to the skillet mixture. Poach for 4 minutes. Turn and poach the other side for 4 minutes.