- Coat 8-inch square baking dish with cooking spray; set aside.
- Preheat broiler.
- Bring medium saucepan of lightly salted water to a boil.
- Add macaroni; stir to prevent sticking.
- Cook until just tender, about 6 minutes.
- Drain well and set aside.
- Meanwhile, in food processor or blender, whirl bread to coarse crumbs.
- Transfer to small bowl.
- Stir in cilantro, salt and mustard and set aside.
- In large saucepan, melt butter over medium heat.
- Add onion, chipotle and scallion and cook, stirring often, until onion is tender, about 4 minutes.
- Stir in flour.
- Cook, stirring, until mixture is fragrant, 1 to 2 minutes.
- Stir in ¼ cup soy milk until no lumps remain.
- Stir in another ½ cup milk until smooth, then remaining ¾ cup milk.
- Increase heat to medium-high and cook, stirring, until mixture boils gently and thickens enough to leave a track on bottom of pot behind spoon, about 2 minutes.
- Stir in "cheese" just until melted.
- Stir in macaroni until coated.
- Transfer macaroni mixture to prepared baking dish and sprinkle with reserved crumbs.
- Broil just until top is browned, about 1½ minutes, watching carefully to prevent burning.
- Serve hot.