To prepare tzatziki: Whisk yogurt, sour cream, lemon juice, garlic and salt in a medium bowl until smooth. Fold in cucumber. Refrigerate.
To prepare rice & cauliflower: Preheat oven to 450 degrees F. Line a large rimmed baking sheet with foil.
Prepare rice according to package directions. Keep warm.
Meanwhile, whisk oil, lemon juice, curry powder and salt in a small bowl.
Remove any outer leaves from cauliflower, but keep stems intact. Place on a cutting board, stem-side down. Using a large chef's knife, cut two 1-inch-thick slices from the center of each head to make 4 cauliflower "steaks." Cut enough of the remaining cauliflower into ¾-inch florets to get 4 cups. Place the steaks and florets in a single layer on the prepared baking sheet. Brush both sides with the curry mixture.
Roast, gently turning halfway through, until the florets are lightly browned and the cauliflower-steak stems are tender, 30 to 35 minutes.
Refrigerate the florets and 6 tablespoons tzatziki for another use (see Tip, below).
Divide the rice among 4 plates and top each portion with a cauliflower steak, ¼ cup tzatziki and a sprinkle of cilantro.
