Mashed Potatoes: Heat butter in a large skillet over medium heat. Add the onions and cook slowly until deep golden color. Add vinegar and thyme and season with salt and pepper, cook for 2 minutes longer. Cook potatoes in a large pot of salted water until soft. Drain well, add butter, heavy cream and horseradish, mash until smooth. Season with salt and pepper. Place potatoes in a bowl and top with the onions.
Squash: Heat oil in a medium saute pan. Add squash and saute until just cooked through. Add butter and cook until melted. Add parsley and season with salt and pepper to taste.
Pan Seared Cod: Preheat oven to 375 degrees F. Heat oil in a large saute pan. Season cod on both sides with salt and pepper. Dredge skin-side in cilantro. Place cod, cilantro-side down in the oil and saute until lightly golden brown. Turn the fish over and place in the oven and continue cooking for 3 to 4 minutes until just cooked through.