Prepare the beef: Mix the olive oil, sliced garlic and marjoram in a baking dish. Add the beef and turn to coat, then cover and marinate at room temperature, turning occasionally, 1 hour.
Meanwhile, make the skordalia: Smash and peel the garlic cloves; warm in a small saucepan with the olive oil over low heat until soft, about 12 minutes. Meanwhile, peel the potato and cut into chunks. Cover with water in a saucepan, season with salt and simmer over medium heat until tender, about 12 minutes. Transfer the potato to a bowl with a slotted spoon, reserving the cooking water, and mash with a potato masher.
Blend the almonds in a food processor until smooth. Add about half of the softened garlic, 1/2 cup of the garlic-flavored oil, 1/3 cup potato cooking water, the vinegar and 3/4 teaspoon salt; puree until smooth. Add the potato and pulse just until creamy. Transfer to a bowl and stir in the marjoram and parsley.
Preheat the oven to 400 degrees F. Remove the beef from the marinade and scrape off any garlic (reserve the marinade). Heat a large ovenproof skillet over medium-high heat. Add 1 tablespoon of the marinade, then sear the beef until browned on all sides, about 8 minutes. Transfer the skillet to the oven and roast until a thermometer inserted into the center of the beef registers 125 degrees, about 15 minutes. Transfer to a cutting board and let rest 10 minutes.
Slice the beef and serve with the skordalia. Top with the remaining garlic oil, softened garlic and more marjoram.