How to Make It
Preheat broiler to high. Scoop out the interior of the baguette and discard. Place hollowed baguette halves, cut sides up, on a baking sheet. Broil until lightly toasted, about 1 minute. Stir together pesto and mayonnaise in a small bowl; spread evenly on baguette halves.
Heat oil in a large skillet over high. Cook sausage, stirring, until browned and crumbly, about 4 minutes. Add spinach to sausage; cook, stirring often, until wilted and bright green, about 2 minutes. Stir in vinegar. Remove from heat.
Transfer toasted baguette halves to a cutting board, and spoon sausage mixture evenly into baguette. Cut each half into 4 equal pieces. Top evenly with cheese. Place on baking sheet; return to oven, and broil until cheese is melted and lightly browned, 1 to 2 minutes.