Preheat oven to 475ºF. Place sausages and onions in a 9-by-13-inch baking dish and toss with oil. Bake until lightly browned, turning once, about 10 minutes.
While sausages are baking, whisk flour and salt in a large bowl. Whisk together eggs, mustard and milk in a large measuring cup, add to flour mixture; whisk until smooth. Carefully pour batter into baking dish with sausages and continue to bake until puffed and golden, about 30 minutes longer. Let stand 5 minutes and serve.