Mix the flour, sea salt, and garlic powder together in a small bowl. Set aside.
In a medium saucepan over medium heat, melt the butter. Add the flour mixture and whisk to combine. Cook for 1 minute, until the mixture is slightly brown.
Add the milk, and whisk until the mixture is smooth. Add the Greek yogurt, and whisk until smooth.
Cook on medium-high heat until the mixture is thickened (about 3-5 minutes). Do not let it boil.
Once the mixture is thick (sticks to the back of the spatula), reduce the heat to low and add the cheese. Whisk until the cheese is melted and the mixture is smooth. Taste and add more salt/seasoning if desired.
Add the cooked pasta, roasted broccoli and peas to the pot of cheese sauce, and stir until the sauce is evenly distributed.
Let the mac and cheese cool for 3-5 minutes or until the cheese sauce has thickened a little bit and sticks to the noodles. Serve warm, and garnish with red pepper flakes, chopped pistachios, nutritional yeast and fresh dill.