Preheat the oven to 350 degrees. In a skillet, heat 1 tbsp. oil over medium. Add the next 3 ingredients, season with salt and pepper and cook, stirring often, until the onion is tender, about 5 minutes. In a large bowl, toss the broccoli with 1 tbsp. oil; season.
In another large bowl, whisk the eggs. Mix in the meat loaf mix, panko, 2/3 cup marinara and the onion mixture; season. On a baking sheet, form into a 6-by-8-inch rectangle. In a bowl, mix the vinegar and 1/3 cup marinara; brush on meat loaf. Bake 35 minutes.
Scatter the broccoli around the meat loaf and bake until cooked through, about 25 minutes more. Let rest 10 minutes.
Meanwhile, in a large pot, cover the potatoes with cold water; bring to a boil. Cook until tender, 15 minutes. Drain, reserving 3/4 cup cooking water. Mash potatoes with cooking water and 2 tbsp. oil; season.
Reserve one-third of the meat loaf for the Three-Cheese Baked Ziti. Slice the remaining meat loaf; serve with broccoli and potatoes drizzled with the remaining 1 tbsp. oil.