Preheat oven to 425°F.

Prepare Yorkshire pudding batter: Whisk together eggs, flour, and salt in a large bowl.

Gradually whisk in milk until smooth.

Fold in chopped thyme.

Transfer batter to a measuring cup.

Cover with plastic wrap and refrigerate for 1 hour.

Place beef on a plate, drizzle with vegetable oil, and season.

Heat a heavy-bottomed, ovenproof frying pan over medium heat.

Sear beef for 1-2 mins per side.

Transfer pan to oven to cook for 15-18 mins for medium-rare.

Transfer beef to a plate and let rest for 10-15 mins.

While meat is cooking, generously brush a 12-cup popover pan with duck fat.

Place pan in oven for at least 8 mins.

When fat is sizzling, fill recesses 2/3 full with batter.

Bake for 10-20 mins, or until golden brown and puffy.

Remove from oven and let rest on a wire rack.

For the sauce, whisk together creme fraiche, mustard, and horseradish.

Slice beef as thinly as possible.

Place a slice of beef on each Yorkshire pudding with a spoonful of sauce.

Garnish with salad greens. Serve hot.