In a small bowl, whisk together 2 tablespoons vinegar, 2 tablespoons olive oil, garlic, rosemary, ½ teaspoon salt, and ¼ teaspoon pepper. Place the steak in a bowl or container; pour in the marinade and turn to coat. Cover and refrigerate for at least 1 hour or up to 6 hours. Remove the steak from the fridge 1 hour before cooking.
Preheat the oven to 425ºF. Line a large baking sheet with parchment paper. In a large bowl, combine the cubed squash, 1 tablespoon olive oil, chili powder, garlic powder, ½ teaspoon salt, and ¼ teaspoon pepper; toss to coat. Spread in a single layer on the baking sheet and roast until tender and caramelized in spots, 35 to 40 minutes, stirring once or twice during roasting time.
Make the dressing: In a small bowl, whisk together remaining 1 tablespoon balsamic vinegar, mustard, miso, maple syrup, 3 tablespoons olive oil, and ¼ teaspoon pepper until well blended. Whisk in hot water 1 teaspoon at a time to thin to desired consistency. Taste and season with salt and additional pepper, if needed. (Yield: About 1/3 cup. You can make the dressing up to 2 days ahead. Cover and keep at room temperature. Whisk well before using.)
Preheat a large cast-iron skillet over medium-high heat until very hot. Remove the steak from the marinade (discard marinade). Pat the steak dry thoroughly; season generously with salt. Swirl the avocado oil in the skillet, then add the steak. Cook undisturbed until seared on one side, 3 to 4 minutes. Carefully flip and cook until medium-rare (an instant-read thermometer stuck into the thickest part should read between 125ºF-130ºF), 3 to 4 minutes longer. Transfer to a cutting board, tent with foil, and let rest for 5 to 10 minutes.
In a large bowl, combine the romaine, cucumber, and radishes. Add 3 tablespoons dressing and a few pinches of salt and pepper; toss to combine. (Toss in more dressing, if desired.) Add the squash; carefully toss. Divide among 4 plates or shallow bowls. Slice the steak against the grain and divide among the salads. Top with pepitas and blue cheese; serve immediately.