Season the oxtails with salt and pepper. Put the flour in a shallow bowl and season with Essence. Dredge the oxtails in the flour, coating each side evenly and tapping off any excess.
Heat 1/2 cup of the olive oil in a large Dutch oven over medium heat. Add the oxtails and cook until very brown, 2 to 3 minutes on each side. Transfer the oxtails to a platter and set side.
Add the onions to the pot, season with salt and pepper, and cook, stirring, for 2 minutes. Add the carrots and celery, season with salt and pepper, and cook, stirring, for 1 minute. Add 2 tablespoons of the garlic, the bay leaves, and thyme and cook for 1 minute. Add the wine and deglaze, scraping the bottom and sides to loosen any browned particles. Add the stock, bring to a boil, and reduce the heat to medium-low. Return the oxtails to the pot and cook, covered, until the sauce is stewlike and the meat starts to fall off the bone, about 2 hours. Baste the oxtails often during the cooking time. Check the seasoning and add salt and pepper if needed. Stir in the parsley.
Meanwhile, put the potatoes in a large saucepan and cover with water. Season the water with salt. Bring to a boil, then reduce the heat to medium and simmer until the potatoes are fork tender, about 10 minutes. Drain. Return the potatoes to the pan over low heat and stir the potatoes for 1 minute to dry them. Add the butter and heavy cream, season with salt and pepper, and mash the potatoes until slightly smooth. Set aside and keep warm.
Heat the remaining 1 tablespoon olive oil in a medium-size sauté pan over medium heat. Add the corn, season with salt and pepper, and cook, stirring, for 2 to 3 minutes. Add the spinach and the remaining 1 tablespoon garlic, season with salt and pepper, and cook for another minute. Fold into the mashed potatoes and keep warm.
To serve, mound the mashed potato mixture in the center of each serving plate. Lay a few of the oxtails on top of the potatoes and spoon some of the gravy over the top.