Make the dressing. Whisk together the horseradish, mayonnaise, sour cream, Dijon mustard, apple cider vinegar, honey, grated garlic, and salt in a small bowl until smooth. Taste — it should be bright with enough kick that you notice it on the first bite but doesn't overpower your palate. Set aside; flavors will deepen after 10 minutes.
Season and sear the steak. Pat the steak dry with paper towels (this is critical for a good crust). Rub with olive oil, then sprinkle generously with kosher salt and pepper. Heat a large skillet over medium-high until it's hot enough that a drop of water sizzles immediately — about 3 minutes. Add the steak and cook undisturbed for 4-5 minutes until well-browned underneath. Flip, reduce heat to medium, add minced garlic around the edges of the pan (it should smell fragrant but not burn), and cook another 3-5 minutes for medium-rare. The internal temperature should read 130°F when you check with an instant-read thermometer — if it reads too high, remember carryover cooking will push it to about 135°F after resting.
Rest the steak. Transfer to a cutting board and let rest undisturbed for at least 5 minutes. This gives the juices time to redistribute so you don't lose them when you slice.
Toss the greens. While the steak rests, combine the mixed greens and shredded red cabbage in a large bowl. Drizzle with about 1 tablespoon of the dressing (save more for serving) and toss gently. The warmth from the resting steak will slightly wilt the greens — this is intentional and delicious.
Slice and assemble. Slice the steak against the grain into thin strips — you want to see no more than a quarter-inch of muscle fibers running through each piece. Pile the steak over the dressed greens, scatter with shaved Parmesan, radishes, shallot rings, and seeds. Drizzle with additional dressing to taste and serve immediately.
