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.