Hey, Is your HV concerned about the weight loss? Has baby started to crawl as their weight tends to dip when they do? Weight gain does slow down and that is totally normal.
As for high calorie cooking, I think a healthy balanced diet is best... I like the Annabel Karmel weaning book for recopies. There are alot of ideas in there and simple to follow. Check your local library or get it from amazon on next day delivery.
That aside, whole milk, butter, avocados, full fat cheese, oil rich fish, eggs are good calorific foods. My DS loves:
scrambled eggs made with whole milk and butter on toast fingers with avocado on the side.
Mashed potatoes, cream cheese and roasted red peppers mashed up together.
Tuna mayo with tiny pasta shapes, with peas and sweetcorn is another firm fave.
Fish pie.
Reduces salt and sugar baked beans with fresh chopped tomato mixed through it on toast with melted cheese on top.
Couscous with roasted up veg.
Grilled steak (fingers) with shallots, mushrooms and spinach.
My DS will at lunch/dinner will have a main dish, as above, and then some fruit and a little yoghurt, either a Rachels organic one or some plain yogurt.
Try BL weaning too, there is no need to smash all the food. A good mix of textures and finger foods at 8m is a good idea.
I HTH.