Season both sides of the ribeye steaks generously with salt and pepper. In a large skillet over medium-high heat, add the olive oil. Once hot, sear the ribeye steaks for about 4-5 minutes on each side until browned. Remove from the skillet and set aside.

In the same skillet, reduce the heat to medium and add the minced garlic and fresh thyme. Sauté for about 1 minute until fragrant. Add the plum preserves and balsamic vinegar to the skillet, stirring to combine. Bring the mixture to a simmer.

Return the ribeye steaks to the skillet, spooning the glaze over them. Cover the skillet and let the steaks braise in the glaze for about 20-25 minutes over low heat, turning halfway through, until the steaks reach your desired doneness.

While the steaks are braising, bring a pot of salted water to a boil. Add the diced celeriac and parsnips. Cook until tender, about 15-20 minutes. Drain and set aside.

In the same pot, add the vegetable or beef broth. Bring to a simmer, then add the cooked celeriac and parsnips back into the pot. Using an immersion blender, blend until smooth. Stir in the heavy cream and butter, cooking until heated through. Season with salt and pepper to taste.

Once the ribeye is done braising, remove from heat and slice against the grain. Serve the sliced ribeye on a plate, drizzled with the plum-balsamic glaze, alongside the velvety celeriac and parsnip gravy. Garnish with fresh thyme sprigs.