Grease a 9x13 baking dish. Preheat your oven to 375°F.
Tear the croissants into large chunks. Layer half in the dish, followed by the ham, sautéed onions, and half of the cheese blend. Top with the remaining croissants.
In a large bowl, whisk the eggs, milk, mustards, 2 tbsp chives, salt, and pepper.
Evenly pour the mixture over the croissants. Press down lightly to submerge. Top with the remaining cheese.
Cover with foil and bake for 25 minutes. Remove foil and bake for another 25 minutes until the cheese is bubbly and the top is golden brown.
Once removed from the oven, top with remaining chives.
Notes
*to caramelize onion, add 1 tbsp oil to a pan over medium heat. Add onions and cook until caramelized.