Preheat a skillet over medium heat and sauté the chopped onion in olive oil until tender.
Remove from heat and let cool for 10 minutes.
In a large mixing bowl, beat the eggs until foamy and well blended.
Add the milk and beat again.
Stir in the flour, Swiss cheese, sugar, pepper, thawed corn, and chopped ham.
Mix until well combined.
Fold in the cooled sautéed onions.
Grease a 3-quart slow cooker and pour in the mixture.
Sprinkle the grated Parmesan cheese on top.
Cover and cook on high for 3–4 hours, or until the casserole is fully set.
Serve immediately while warm.
Notes / Tips / Wine Advice:
Serving Tip:For a touch of sweetness, drizzle with warmed maple syrup before serving.Wine Advice:Pair with a light Chardonnay or a fruity Riesling to complement the sweet and savory flavors.