For the pickled onions
- 1 small red onion, halved and thinly sliced crosswise
- Kosher sea salt
- 1-1/2 tsp. granulated sugar
- 1/4 cup distilled white vinegar
For the tartare
- 5 small beets (about 13 oz.)
- 7-1/2 Tbs. extra-virgin olive oil
- Kosher sea salt and freshly ground black pepper
- 3/4 lb. ground or very finely chopped raw sirloin, preferably dry-aged (ask your butcher to grind)
- 3 Tbs. minced white onion
- 2 Tbs. capers, drained, rinsed, and coarsely chopped
- 1-1/2 Tbs. finely chopped cornichons
- 1 Tbs. finely chopped fresh flat-leaf parsley
- 1 tsp. minced anchovy fillet
- 1 large egg yolk
- 8 1/2-inch-thick slices focaccia, lightly toasted
- Fleur de sel, for finishing
- Small edible flowers (optional)
Make the pickled onions
- In a medium bowl, toss the onion with 2 Tbs. salt and the sugar. Let the mixture sit at room temperature until the onion begins to soften, 15 to 20 minutes. Transfer to a fine-mesh strainer and rinse under cool water while tossing with your fingers to remove the salt and sugar. Pat dry with paper towels. In a small bowl, combine the onion and vinegar. Cover and refrigerate until needed.
Make the tartare
- Position a rack in the center of the oven and heat to 400°F.
- Pierce the beets in several places with a fork. Put the beets in a foil-lined small rimmed baking dish. Drizzle with 2 Tbs. of the oil and sprinkle with 1/2 tsp. salt and 1/2 tsp. pepper. Cover with foil and roast until the beets are very tender, about 1 hour. Peel and quarter when cool to the touch. Put the beets in a food processor with 1/4 tsp. salt and a few grinds of pepper. Pulse several times until the beets are chopped into small pieces. With the motor running, drizzle in 4 Tbs. of the oil and process until mostly smooth, scraping down the bowl occasionally, about 2 minutes. Season to taste with salt and pepper, and set aside.
- In a medium bowl, combine the sirloin, onion, capers, cornichons, parsley, anchovy, egg yolk, 1/2 tsp. salt, 1/4 tsp. pepper, and the remaining 1-1/2 Tbs. of the oil. Fold together gently to combine. Do not overmix, or the sirloin will toughen. Season to taste with salt and pepper, if necessary.
- Spread some of the beet purée onto each slice of focaccia. Top with some of the tartare, pickled onion, fleur de sel, and an edible flower, if using. Serve immediately.