marinated sliced lamb with sauce

Name: marinated sliced lamb with sauce
Ingredients: 12 pieces lamb (strips); 2 tablespoons ketchup; 1 teaspoon brown sugar; 1/2 teaspoon cumin; 1/2 teaspoon dried coriander; 1/4 teaspoon ground pepper; 1 teaspoon dijan mustard; 1 tablespoon soy sauce; 1 tablespoon rice vinegar or apple vinegar; 1/4 teaspoon lemon zest or lime zest; 1 tablespoon butter; 4 tablespoons lemon juice; 1 cup water; 1 pinch salt; 2 tablespoons olive oil or veg oil; 1 small minced onion; 1 large minced clove garlic