Step 1 In a medium bowl, combine 1 1/2 pounds heirloom tomatoes, coarsely chopped and juices reserved. Add their juices with 2 tbsp minced shallots, 1 clove garlic minced, 1 tbsp white balsamic vinegar, 1 tbsp snipped chives, 1 tbsp chopped flat-leaf parsley, 2 tbsp olive oil and 1/4 tsp ground fennel seeds. Season with salt and pepper.
Step 2 In a small bowl, mix 3/4 tsp of ground fennel with 1/2 tsp each of salt and pepper. Sprinkle the mixture all over the fish. In a medium nonstick skillet, heat 1 tbsp of olive oil until shimmering. Add the halibut and cook over moderately high heat until browned on the bottom, about 5 minutes. Flip the fillets and add 1 tbsp unsalted butter to the skillet; spoon the butter over the fillets as they cook, about 2 minutes longer. Transfer the halibut to plates, spoon the tomato vinaigrette on top and serve.