Combine the vinegar, sugar, caraway seeds, 1 teaspoon kosher salt, and 1/4 cup water in a small pot. Bring to a boil over high heat. Remove from heat. Add the cucumber and fennel. Let sit, tossing occasionally, until cool, 20 to 30 minutes; drain.
Top the smoked salmon with the watercress or frisée and the pickled vegetables. Serve drizzled with olive oil and topped with flaky sea salt. Make and refrigerate the pickled vegetables up to 4 days in advance.
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