Octopus Salad With Pickled Lemons

Ingredients

• 2⁄3 cup white wine vinegar
• 1⁄2 cup packed brown sugar
• 1⁄4 tsp. kosher salt
• 1 lemon, thinly sliced and seeded
• 3 tbsp. plus 2 tsp. grapeseed oil
• 3 tbsp. plus 2 tsp. white wine vinegar
• 1 1⁄2 tsp. whole-grain mustard
• 1 tsp. honey
• 1⁄4 cup kosher salt
• 2 lb. octopus (about 4 large tentacles), cleaned
• 1⁄2 cup extra-virgin olive oil
• 14 cups mixed baby salad greens such as spinach and arugula (7 oz.)
• 1 1⁄2 cups fennel fronds, chopped (from 2 bulbs)
• 2 cups cherry tomatoes, large ones halved
• 3⁄4 cup thinly sliced spring onion
• 1⁄2 tsp. flaky sea salt