In a bowl, place the sun-dried tomatoes. Cover them with boiling water and set aside for about 10 minutes. Adjust the amount of water to cover all the tomatoes. In the meantime, start preparing the other ingredients.

Cut a slice from the peeled onion and finely chop it. Wash, dry, and slice the mushrooms.

Heat half of the butter in a pan. After a moment, add the onion and fry for a few minutes, stirring occasionally. Then add the mushrooms. Fry for a few more minutes, stirring occasionally.

Drain the tomatoes (preferably on a sieve) and cut them into smaller pieces.

Transfer the sautéed ingredients to a tall container, such as a blender jar. Add the sun-dried tomatoes, peeled garlic, and olive oil. Season to taste with salt, pepper, and oregano. Blend until smooth.

Lay the meat flat on a cutting board. Carefully cut it in half lengthwise (from one side) with a sharp knife, making sure not to cut all the way through. Spread the cut meat flat and gently pound it with a mallet or large spoon (be careful not to damage the meat—it should not have holes). You should obtain a thin, flat piece that can be rolled up.

Evenly spread the mushroom and sun-dried tomato mixture on the prepared piece of meat. Roll up the roulade.

Heat the remaining butter in a pan. Add the roulade (seam side down) and gently fry on each side. Do not fully cook, just lightly brown. If necessary, use toothpicks to prevent the roulade from unrolling. Preheat the oven to 180 degrees Celsius.

Transfer the roulade to an oven-safe dish and bake covered for about 15-20 minutes, depending on the oven's strength.

Place on a plate.