Place cauliflower in the bowl of a food processor; pulse until it resembles the texture of quinoa (you should have about 2 cups.)
In a large bowl, whisk together lemon zest and juice, mustard, 1 1/2 teaspoon salt, and 1/4 teaspoon pepper. Gradually add oil and whisk until thick and emulsified. Add cauliflower rice, cucumbers, tomatoes, and scallions. Season with salt and pepper, to taste. Cover and let stand at room temperature for at least 30 minutes or refrigerated for up to 1 day. Before serving, stir in parsley, mint, and feta; adjust seasoning.