Season the steak with 1 teaspoon salt and a few grinds of pepper. Transfer to a covered container, toss with the garlic and 2 tablespoons of the olive oil, and chill, covered, for at least 2 hours.
Place the potatoes in a small saucepan, cover with water, season generously with salt, bring to a gentle boil, and cook until fork-tender, 10 to 15 minutes. Drain and set aside to cool.
Preheat the grill to medium-high. Brush the garlic from the steak, then grill 3 minutes per side for medium rare. Transfer the steak to a cutting board, let rest for 5 minutes, then slice thinly against the grain.
Halve the potatoes lengthwise. In a large bowl, toss the potatoes with the arugula, 1/8 teaspoon salt, the remaining 2 tablespoons olive oil, and the lemon juice. Transfer to a serving platter, add the avocado slices, sprinkle with salt, top with the steak, and serve with the salsa.
.Line a cast-iron skillet with aluminum foil. Heat the skillet over high heat for 3 minutes. Add the tomatillos, onion, garlic, and jalapeño and cook, turning each ingredient occasionally, until blackened on all sides and soft, about 10 minutes for the garlic, onion, and jalapeño and about 20 minutes for the tomatillos. Stem and seed the jalapeño, peel the garlic, and stem the tomatillos.
Pulse all of the charred vegetables in a blender until finely chopped or blend until smooth, if desired. Transfer to a bowl, stir in 1/4 teaspoon salt, the cilantro, and the lime juice, and serve.