Think you have the right approach, OP - you need to expand her diet in a firm but fair way, but you also need to keep the emotion out of it if at all possible.
Will your CM do a cooked lunch, or a sandwiches/bits on a plate type thing? Win for you if it's a cooked lunch, as you won't need to fuss so much over dinner!
Do watch out for her filling up on breakfast after going hungrier the evening before - this can easily become a habit, especially if she's strong-willed. If she knows she can always increase the breakfast foods in the morning, then she'll fill up then and not be hungry for lunch and it becomes a vicious cycle.
Things to try, maybe:
Swap Weetabix at breakfast for Oatibix (hopefully to lead on to porridge etc. eventually), have one day at least where you offer something totally different to the favourite
Soups with bread/toast to dip in (add to cheese to sprinkle, start with tomato soup or another flavour she likes then add red lentils etc for )
Things in pancakes (banana and peanut butter?)/savoury pancakes (can hide cream cheese for protein, is quite bland)
French toast, as mentioned by lots of people - make an apricot jam sandwich then dip in egg and fry, this could be a good one to do with her so she can see what's gone into it.
Tortilla pancakes big hit with mine - make quesadillas (can be as simple as ham/cheese/tomato puree then dry fry) or add your own filling fajita-style with lots of bits on the table.
If she likes most fruit, then tomatoes, peppers, carrots, peas and sweetcorn, plus of course sweet potatoes should appeal to her sweet tooth.
Basically you want to encourage her to eat what you guys have, but no pressure initially if she doesn't try. So serve lasagne with bread and a salad, if she only eats the bread then OK. After a while she'll get bored and try it.
I also agree with someone upthread who says get her to choose something from the shop/greengrocer/supermarket herself just to try - amazing what that will do sometimes.
Good luck.