Prepare the Filling:Heat olive oil and butter in a skillet over medium heat. Add chopped onion and cook until translucent. Add garlic and mushrooms, cooking until the mushrooms release their moisture and start to brown. Add chopped spinach and cook until wilted. Season with salt, pepper, and nutmeg, then remove from heat. Stir in cheese and set aside to cool slightly.

Make the Meatloaf Mixture:In a large bowl, combine breadcrumbs and milk. Let sit for 5 minutes so the breadcrumbs absorb the milk. Add the ground meat, eggs, onion, garlic, ketchup, Worcestershire sauce, mustard, salt, pepper, and thyme. Mix gently with your hands until well combined but not overworked — overmixing can make the meatloaf dense.

Assemble the Meatloaf:Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper or lightly oil a loaf pan. Spread the meat mixture out on a piece of parchment paper into a rectangle about ½ inch thick (roughly 10x12 inches / 25x30 cm). Spoon the spinach and mushroom filling evenly over the surface, leaving a small border around the edges. Carefully roll the meatloaf from one short end to the other, jelly-roll style, using the parchment to help lift it. Seal the edges and ends to keep the filling inside.

Add the Glaze:Place the rolled meatloaf seam-side down on the prepared baking sheet or pan. Mix ketchup, brown sugar, and mustard in a small bowl, then spread evenly over the top.

Bake:Bake uncovered for 50–60 minutes, or until the internal temperature reaches 160°F (70°C). If the glaze starts to brown too quickly, cover loosely with foil. Let rest for 10 minutes before slicing — this allows the juices to redistribute and keeps your slices neat.

Serve:Slice the meatloaf into thick pieces to reveal the beautiful spiral of spinach and mushrooms. Serve with mashed potatoes, roasted vegetables, or a light salad.