1 1/2 tsp fresh lemon juice

Pat steak dry and sprinkle all over with a scant 1/2 tsp salt and 1/4 tsp pepper.

Heat oil and butter in a 10-inch heavy skillet over medium heat until foam subsides, then cook steak, turning once, until well browned, 7 to 10 minutes total for medium-rare. Transfer to a cutting board and let stand, uncovered, 10 minutes.

Add shallot to skillet and cook, stirring, until softened, about 3 minutes. Add cream, blue cheese, and spinach and cook, scraping up brown bits from bottom of skillet and turning spinach with tongs, until spinach is wilted, about 2 minutes.

Stir in lemon juice and salt and pepper to taste. Thinly slice steak and serve with spinach.