Using a cast iron enamel cast iron roaster with a lid like Le Creuset or a Dutch oven, melt the butter in the olive oil.
Salt and pepper the lamb fillets. Sauté them for 2 minutes on each side.
Add the thyme, cover the pot and continue to cook for another 3-4 minutes.
Take the meat and thyme out and reserve on a hot plate in a warm place. Deglaze the meat juices with the white wine.
Add the small diced vegetables and cook another 5-6 minutes while stirring until tender.
In a separate frying pan, sauté your spinach in melted butter until tender.
When ready to serve, place the sautéed spinach on the lower portion of a hot plate; with a slotted spoon, top the cooked spinach nicely with the sautéed vegetables (drain and save the sauce).
Place the lamb filets on the top portion of the plate and coat with the sauce and Voila!