Preheat oven to 400°F (200°C). Line two baking sheets with parchment paper. Pulse half the cauliflower florets in food processor into very finely chopped pieces resembling rice. Transfer to large microwaveable bowl; repeat process with remaining cauliflower. Cover bowl with plastic wrap and microwave on HIGH 5 to 6 min., or until softened. Spread cauliflower onto a clean cheesecloth or tea towel; cool about 5 min. Wrap cloth around cauliflower and squeeze out excess moisture.
Transfer dried cauliflower into large, clean bowl. Mix with eggs, cheese, flaxseeds, ½ tsp (2 mL) salt and ¼ tsp (1 mL) pepper to form crust mixture. Pour onto one of the lined baking sheets. Using back of a large spoon, spread mixture into 12 x 8-in. (30 x 20-cm) rectangle.
In medium bowl, toss sweet potatoes with vegetable oil and remaining ¼ tsp (1 mL) each salt and pepper. Spread sweet potatoes over ¾ of the second lined baking sheet and place chicken sausage on remainder of sheet.
Bake cauliflower crust, sweet potatoes and chicken sausage 20 to 25 min., turning sweet potatoes and sausage halfway, until sausage is cooked through and sweet potato slices are tender. Cauliflower crust should be golden brown and crisp at edges (keep oven on). Cool crust 5 min. on baking sheet.
Meanwhile, cut sausage into ¼-in. (5-mm) rounds. In bowl, mix ricotta with parsley and spread evenly over cauliflower crust. Top with sweet potato and sausage. Bake 10 min., or until ricotta and vegetables are warmed through. Sprinkle with arugula before serving.
Replace cauliflower with equal amount of broccoli for a broccoli pizza crust. Omit chicken sausage for a vegetarian option