2 large fennel bulbs, trimmed and cored, about 4 cups, plus a few fronds, if any, for garnish
2 cups water
1 teaspoon sea salt or coarse kosher salt
1 teaspoon coriander seeds
2 teaspoons fennel seeds
1/2 teaspoon brown mustard seed
1/2 teaspoon whole black peppercorns
3 tablespoons olive oil plus more for garnish
1 small yellow onion, cut into 1/4 inch (dice - about 1/2 cup)
2 teaspoons minced fresh garlic
1 tablespoon fresh lemon juice
1 tablespoon balsamic vinegar
1   28 ounce can crushed Italian plum tomatoes (preferably San Marzano)
    Freshly ground black pepper