Preheat oven to 425°F. Rub potatoes with oil and salt, place on a parchment-lined sheet, and bake for 50–60 minutes until tender.
Trim and cut steak into 2-inch chunks. Toss with oil, salt, and Cajun seasoning.
Heat oil in a skillet over medium-high. Cook steak 2 minutes without stirring, flip, then cook 2 more minutes.
Push steak to one side. Add butter and garlic; sauté until fragrant. Toss steak to coat, cook 1 more minute, then transfer to a bowl and cover.
In the same skillet, melt butter and sauté garlic. Whisk in cream, simmer 3–5 minutes. Stir in red pepper flakes, Parmesan, and pepper until thick. Remove from heat, add parsley and lemon juice.
Drop baked potatoes to fluff. Slice, mash lightly, add butter, top with steak, and pour over sauce. Serve hot.