How to Make It
Place black garlic in a small bowl, and mash with a fork to a smooth paste. Add butter, brandy, and 1/4 teaspoon salt; mash until thoroughly combined. Spoon garlic mixture onto a sheet of parchment paper or plastic wrap, and roll into a 3-inch-long log. Refrigerate until ready to use.
Mound spiralized potatoes on a large microwavable plate or in a glass pie plate; cover with wet paper towels. Microwave on high until tender, 4 to 6 minutes. Spread in an even layer on a rimmed baking sheet, and let cool 10 minutes.
Pour peanut oil to a depth of 1 1/2 inches in a large, deep Dutch oven; heat over medium to 375°F. (The hot oil will rise when potatoes are added.)
Meanwhile, heat 1 tablespoon peanut oil in a 12-inch skillet over medium-high. Pat steaks dry with paper towels; sprinkle with pepper and remaining 2 teaspoons salt. Add steaks to skillet, and cook, turning every 3 minutes to evenly brown all sides, to desired degree of doneness, about 12 minutes for medium-rare. Transfer steaks to a carving board, and let rest 10 minutes.
Meanwhile, fry potatoes in hot oil in 2 to 3 batches, stirring often with a spider or slotted spoon, until frites are golden and crisp, 4 to 6 minutes. (Adjust heat between batches as needed to maintain oil temperature of 375°F.) Transfer frites to a butcher paper–lined plate. Sprinkle with salt.
Cut steaks against the grain into slices; divide among 4 plates. Cut black garlic butter into 1/2-inch-thick rounds. Top steak with black garlic butter, and, if desired, parsley. Serve immediately with frites.
Roasted garlic may be substituted for black garlic.