Tangy Brisket With Fennel and Herbs

Ingredients

• 1 4–5-lb. piece untrimmed flat-cut beef brisket
• Kosher salt
• Freshly ground black pepper
• 4 Tbsp. vegetable oil, divided
• 1 medium onion, cut through root end into 1"-thick wedges
• 1 large fennel bulb, cut through root end into 1"-thick wedges
• 3 celery stalks, cut into 2" pieces, plus 1 cup leaves
• 1 head of garlic, halved crosswise
• ½ bunch thyme, oregano, or marjoram
• ¾ cup distilled white vinegar
• ¼ cup low-sodium soy sauce or Worcestershire sauce
• 2 cups mixed tender herbs (such as parsley, mint, and/or cilantro)
• ½ lemon
• Crushed red pepper flakes (optional)
• Flaky sea salt