8 garlic cloves, smashed and peeled
3 medium fennel bulbs, trimmed, reserving some fronds for garnish, and bulbs (including core) cut lengthwise into 1/4-inch-thick slices
1 1/2 pound small (1 1/2- to 2-inch) tomatoes (preferably Campari), halved
1 teaspoon fennel seeds
1 teaspoon sugar
1 Turkish or 1/2 California bay leaf
3 (3- by 1-inch) strips orange zest
1 quart extra-virgin olive oil
1 (2 1/2-pound) piece skinless halibut fillet (about 1 1/2 inches thick)
a 5- to 6-quart heavy pot (at least 4 inches deep)
Simmer garlic, fennel bulbs, tomatoes, fennel seeds, sugar, bay leaf, zest, and 1 teaspoon salt in oil, stirring occasionally, until vegetables are tender but still intact, 30 to 40 minutes.
Vegetables and fish can be cooked 1 day ahead. Remove from oil to cool, then chill in oil, covered. Reheat to warm over low heat, 10 to 15 minutes, before removing from oil.