Whisk together whipping cream and hollandaise sauce mix in a small bowl just until combined; set aside.

Whisk together eggs, ½ cup hollandaise sauce mixture, 1½ tablespoons ground mustard, 1 tablespoon smoked paprika, and ½ teaspoon salt in a medium bowl. Stir in tomatoes and red onion; set aside.

Toss hash browns with remaining ½ teaspoon salt in another medium bowl. To assemble, spread half of the hash browns in bottom of a 6-quart slow cooker. Layer with half of the egg mixture, ¾ cup Cheddar cheese, half of the Canadian bacon, ¾ cup spinach, and 2 tablespoons hollandaise sauce mixture. Sprinkle with remaining ½ tablespoon dry mustard. Repeat layers. Cover and refrigerate remaining hollandaise sauce mixture for serving.

Cover and cook casserole on high for 3 to 3½ hours or low for 5 to 6 hours or until egg mixture is set. Sprinkle with remaining ½ cup Cheddar cheese. Let stand, uncovered, for 10 to 15 minutes before serving.

Whisk together remaining hollandaise sauce mixture and milk in a small microwave-safe bowl. Microwave on high for 1 to 2 minutes or until mixture comes to a boil. Whisk until smooth.

To serve, cut casserole into serving pieces. Drizzle with hollandaise sauce mixture. Garnish with additional smoked paprika and chives, if desired.