What to serve (vegetarian) with mashed potatoes?
When I was a vegetarian, I used to mix spinach and roasted garlic in with my mashed potatoes to make them a bit healthier (I still love them this way), and then serve that with some sauteed zucchini with thyme. If you're going for a bit more rustic you could serve them with some roasted aparagus or roasted red onions/shallots with a balsamic glaze.
something with portobellos. brush mushrooms with olive oil and balsamic and grill. Or dry roast mushrooms in the oven. Or pan saute a variety with shallot and a bit of thyme in butter.
you could do a fried eggplant in the style of chicken fried steak. (or like eggplant parmesan).
What about something like a vegetarian shepherd's pie? Something along these lines (below) but you could make it really simple if you sauteed portabello mushrooms in tomatoes and onions as a bottom layer, corn above and then mashed potatoes.
Tempeh is a largely ignored but vegetarian and delicious food! Simply cube and saute in oil -- dark sesame is great -- and/or in butter if you've got a lactovegetarian there. And goes really well with mashed potatos. Typical tempeh flavorings (for me) include garlic, mustard, and cumin but it's a very versatile Indonesian food.Tempeh is a largely ignored but vegetarian and delicious food! Simply cube and saute in oil -- dark sesame is great -- and/or in butter if you've got a lactovegetarian there. And goes really well with mashed potatos. Typical tempeh flavorings (for me) include garlic, mustard, and cumin but it's a very versatile Indonesian food.
Tempeh goes well with arame or hijiki sea vegetables. One brand sells the tempeh with it already inside.
I like to mix roasted corn & garlic and peas into my mashed potatos.
I assume you are looking for an entree to accompany side (potatoes) and salad? If I understand, I agree with cocktailhour's suggestion for a portabello -- I usually rub mine with seasoning like a steak and then cook caps in a pan with balsamic (about 1/4 cup for 2 large caps) until balsamic reduces. Then melt cheese (something smokey) on top -- it's like having a steak.
Alternately, Mark Bittman's NYT article a while back with 101 10-minute recipes included an easy tofu prep that would go well with potatoes: coat cubes of extra-firmm tofu in thin layer of flour, pan fry with a little oil (I use Pam), add ketchup and continue cooking in pan until reduced. This would cover your protein element. Of course, there are many ways to prepare tofu; I prefer Asian-style, but that's not a great accompaniment to mashed potatoes.