Bring a large pot of salted water to a boil. Cook the macaroni according to package directions. Drain and reserve.
Heat a large sauté pan over medium-high heat. Add the olive oil and sauté the ground beef, breaking it apart with a spatula until lightly browned (about 5 minutes).
Add the onion, carrot, celery and oregano. Cook until softened (about 2 minutes).
Stir in the garlic and cook until aromatic (about 30 seconds). Stir in the Swiss chard and yellow squash. Cook to wilt the chard (about 1 minute). Sprinkle in 2 Tbsp. of the flour and stir to combine.
Stir in the stock and heat through and allow to thicken slightly (3 to 5 minutes). Taste and adjust seasoning with salt and pepper. Reserve.
Preheat oven to 350ºF. In a medium saucepan, whisk together the butter and remaining 2 Tbsp. flour until butter foams. Stir in milk and nutmeg.
Continue stirring until the béchamel thickens. Season with salt and pepper. Stir in the reserved macaroni.
Place the reserved meat mixture in a 9”x9” pan. Top with macaroni. Place in the oven and cook until heat through and golden on top (about 20 minutes).