Preheat the oven to 350F.

In a large saucepan, bring the water, milk and butter to a boil, then add 1/2 tsp salt to the mixture.

Quickly whisk the polenta into the mixture, stirring constantly until any lumps are gone (about 4 minutes).

Reduce heat and let the polenta simmer, partially covered, stirring every 10 minutes, until the polenta is smooth and thick (about 45 minutes).

While the polenta is cooking, lightly grease a sheet pan, cut the tomatoes in half and arrange them cut side down on the pan. Cut the ends off of the asparagus and add them to the pan.

Drizzle olive oil over the vegetables and season with 1 tbsp chopped rosemary, 1 tsp thyme, salt and pepper.

Bake at 350F until the tomatoes are very tender (about 20-25 minutes). Set aside 3 tomato halves for the vinaigrette and cover the pan with foil to keep warm.

For the vinaigrette, process 3 tomato halves, 1 1/2 tbsp olive oil, vinegar, 1/8 tsp salt and 1/8 tsp pepper in a food processor until smooth.

Once the polenta is thick and smooth, add the Creme Fraiche (or sour cream) and parmesan, then stir until smooth. Season to taste with salt and pepper and keep warm over low heat.

Season each fillet of Mahi Mahi with salt, pepper, thyme and chopped rosemary.

Heat 1 tbsp olive oil in a saute pan and sear the fish until cooked through (about 3 minutes per side).

Split the polenta between two dishes, top each with a fillet of Mahi Mahi and drizzle the tomato vinaigrette over each. Place asparagus and roasted tomato halves around the fish and serve.