Preheat the oven to 200 degrees celsius. Place the wire rack in the center of the oven.
Break eggs in a medium bowl, add milk, cream, mozzarella, salt and pepper and whisk together till the eggs are just broken.
Warm oil in a 10" skillet (preferably cast iron or an oven proof skillet). Saute onions, garlic, broccoli, bell peppers and zucchini for 5-7 minutes, till softened. Add the potatoes, spring onions and spinach and give it a stir. Cover and cook for a couple of minutes till the greens are wilted.
Distribute the vegetables evenly to cover the bottom of the skillet, add a layer of grated mozzarella and pour the egg mixture into the skillet over the vegetables and cook on a slow flame for a couple of minutes until the eggs are just beginning to set around the edges.
Sprinkle crumbled feta and parmesan over the egg mixture and place the skillet in the oven till the eggs are set, about 20-22 minutes (start checking around the 15 minutes mark - you can insert a fork in the middle to see if the eggs are cooked). Once set, switch the oven setting to grill (or broil) and grill the top till golden, about 2 minutes.
Remove from the oven and let the frittata sit for 3-4 minutes before slicing. Serve the frittata with a salad on the side.