1- Preheat oven to 350F.
2- In a frying pan over medium heat, melt butter and saute onions.
3- Add garlic, carrots and mustard powder when onions are translucent.
4- When carrots are soft, remove from heat.
5- In a casserole dish pour in salmon, onion mixture, olives, feta cheese, and lemon juice. Mix well so that salmon is flaked apart, and spread evenly.
6- Top with mashed potatoes.
7- Put in oven to heat through salmon (everything is already cooked at this point). If desired, broil until potatoes are golden brown on top.