In casserole over medium heat, cook garlic and sausage, breaking up meat, about 5 minutes; transfer to a bowl with a slotted spoon. Drain all but 1 tablespoon fat; cook kielbasa until crisp, 8 to 10 minutes. Add to bowl.
Arrange potatoes in pan, cut side down; cook until golden brown, 5 to 7 minutes. Turn; cook about 5 more minutes, until just tender. Spread fennel among potatoes. Cook about 10 more minutes, stirring often.
Add leek, Pernod, clam juice, salt, and pepper; cook until vegetables are tender, about 5 minutes. Return sausages to pan; mix to combine. Add clams, cover, and cook about 5 minutes. Add tomatoes; cook, covered, until clams open, about 8 minutes; discard unopened clams. Stir in tarragon, and serve.