Unbaked macaroni and cheese can be refrigerated up to 2 days in advance until ready to bake.
1) Preheat the oven to 350 degrees F. Lightly grease a 9- x13-inch pan.
2) Bring a large pot of heavily salted water to a boil. Add macaroni and cook according to package directions. Drain and return to pot.
3) Meanwhile, in a medium saucepan, make a roux or paste by whisking the margarine and flour over medium heat for 3 to 5 minutes. Add nondairy milk, yeast, tomato paste, salt, and garlic powder to the saucepan and bring to a boil, whisking frequently.
4) Reduce heat to low and let simmer until the sauce thickens. Adjust seasoning to taste and stir in lemon juice and agave. Toss noodles with the sauce and transfer to the prepared pan.
5) Sprinkle breadcrumbs on top of the pasta and bake for 30 minutes, or until the top is lightly browned and crisp. Remove from oven and let rest for 5 minutes before serving.