For the Mustard Vinaigrette, in a medium bowl, combine the honey and mustard powder. Blend with a fork until well mixed, then let stand for 10 minutes to bloom the mustard. Whisk in the lemon juice, whole-grain mustard, Dijon mustard, and salt. Slowly whisk in the olive oil until emulsified. The dressing can be made up to 2 days ahead and refrigerated until ready to use.
For the Pickled Red Finger Chiles, thinly slice the chiles crosswise and discard the stems. Place in a small sterilized jar or storage container and cover with red wine vinegar. Cover and refrigerate for at least 12 hours before using. The pickled chiles will keep for about 2 weeks.
Bring a large saucepan of salted water to a boil and prepare an ice bath. Working in batches, add the Brussels sprouts and blanch until slightly tender, 2 to 3 minutes. Transfer to the ice bath to cool, then transfer to paper towels to drain. Once all of the sprouts have been blanched, cut them in half lengthwise and pat dry with towels.
In a large skillet, heat the oil over medium-high heat. Working in batches if necessary (don’t crowd the pan), add the sprouts, cut-side down, and cook until well browned on the bottom, 5 to 7 minutes. Stir and continue cooking until tender and caramelized all over, about 5 minutes longer. Return all of the sprouts to the pan and add the butter and rosemary, tossing until the butter has melted and coated the sprouts.
Add enough mustard vinaigrette to cover the bottom of a serving bowl (about ⅓ cup). Top with the sprouts and sprinkle with the pickled onions and 1 tablespoon finely chopped pickled finger chiles. Season with flaky salt and serve.
