For the dip: Combine the cream cheese, sour cream, horseradish, lemon juice, lemon zest, granulated garlic, 1/4 teaspoon salt and a generous grinding of black pepper in the bowl of a food processor. Process until well combined and smooth. Transfer the dip to a medium bowl and fold in the chives and parsley. Cover and refrigerate until ready to serve.
Bring a large pot of salted water to boil and prepare an ice bath by filling a large bowl with ice water. Add the broccoli to the boiling water and blanch until bright green and crisp tender, 2 to 3 minutes. Remove with a strainer to the ice bath. Repeat with the cauliflower. Drain the vegetables well and pat them very dry.
To build the tree, make a triangle of broccoli florets in the center of a serving platter. Place the cucumber piece, skin side up, at the base of the tree as a stump and scatter the cauliflower on either side. Decorate the tree with tomatoes, cucumber slices and pepper strips and stars, leaving one star for the top.
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