Preheat and prep: Heat the oven to 375°F (190°C). Grease a 9x13-inch baking dish or a large oven-safe skillet. If your veggies are raw and sturdy (like broccoli or carrots), chop them small so they cook through.
Sauté the base: In a large skillet, melt butter over medium heat.
Add onion and a pinch of salt. Cook 3–4 minutes until soft. Stir in garlic for 30 seconds until fragrant.
Make a quick sauce: Sprinkle in flour and stir for 1 minute.
Slowly whisk in milk or broth until smooth. Add Dijon and thyme. Simmer 2–3 minutes until slightly thickened.
Season with salt and pepper.
Add the good stuff: Stir in ham, mixed vegetables, and your cooked starch. If using leafy greens like spinach, add them now so they wilt. Taste and adjust seasoning.
Cheese and assemble: Stir in half the cheese.
Transfer to your baking dish if not using an oven-safe skillet. Sprinkle the rest of the cheese on top. Add breadcrumbs or crushed crackers if you like a crunchy finish.
Bake: Bake 18–25 minutes, until bubbly around the edges and lightly golden on top.
If your top needs extra color, broil for 1–2 minutes—watch closely.
Finish and serve: Let rest 5 minutes to set. Top with chopped parsley or green onions for freshness. Serve with a simple salad or sliced fruit.