Aged Rib Eye with Onion Purée

Ingredients

• 1 tablespoon buttermilk 
• 4 servings sea salt 
• 4 servings kosher salt 
• 0.3 cup chicken stock see 
• 3 cups onion packed very thinly sliced ()
• 28 ounces beef rib steak thick for 1 hour at room temperature ( 2" )
• 1 sprigs flat parsley assorted soft (such as tarragon, flat-leaf parsley, and chervil)
• 6 tablespoons butter unsalted divided
• 1 tablespoon vegetable oil 

Equipment

• frying pan
• baking sheet
• oven
• blender

Directions

1. Heat 2 tablespoons butter in a 12" heavy skillet overmedium-low heat.
2. Add onions and cook,stirring constantly, until translucent (do notbrown), 10-12 minutes.
3. Add stock and 3/4 cupwater. Bring to a simmer, reduce heat to low,cover, and continue to simmer until onionsare falling apart, about 20 minutes. Uncoverand stir until onions are almost dry (do notbrown), about 5 minutes.
4. Transfer onions toa blender.
5. Add buttermilk and 1 tablespoon water.Purée until smooth. Season with kosher salt.
6. Preheat oven to 350°F. Arrange a wire rackinside a rimmed baking sheet. Melt 2 tablespoonsbutter with oil in a large cast-iron skillet orlarge heavy skillet over medium-high heat.Season steak generously with kosher salt.Cook until a deep brown crust forms, 3–4minutes per side and 1–2 minutes on edges.
7. Place on prepared rack. Roast until a thermometerinserted into steak registers 115°F,about 40 minutes. (Steak will carry over tomedium-rare.)
8. Let rest for 30 minutes.
9. Melt remaining 2 tablespoons butter in a largeskillet over medium-high heat; cook untilbutter browns, about 2 minutes.
10. Add steakand cook for 30 seconds per side, allowingsteak to absorb butter.
11. Cut steak into 4slices.
12. Place 1 steak slice on each plate andsprinkle with sea salt. Spoon warm onionpurée alongside. Scatter herbs over purée.