Cashew nuts are wonderfully calorific. A great snack. But tbh any nuts or a mix are perfect for this. Add some dark (70, 80% +) chocolate too.
Homemade pesto for pasta/potato salad/a dressing for hot veg. Pine nuts are very nutritious and high in calories, blend a whole packet with plenty of olive oil and then a herb/garlic etc of your choice and drizzle away.
Make bread and load it with tons of seeds. If you aren't so keen on the texture then you can whizz up a mix of seeds in a chopper/processor and pour them in as extra flour.
Add a couple of scoops of ground almonds to your porridge. Top with whole nuts/seeds and a slosh of cream, and drizzle of honey.
I saw a tip once to fortify your milk to increase calories - basically add a couple of scoops of dried milk to normal milk and shake to dissolve. Use milk instead of water when making up soups and sauces.
Generous olive oil/butter to fry in or dress veg etc with.
Coffee with cream float your boat?
I am very partial to a banana sliced lengthways, then filled with 1-2 tbsp peanut butter, sandwiched back together and eat.
Hummous can be fairly heavy on the calories, esp if you make it yourself as you can be generous with the oil. A nice dip for veg/pitta bread/tortillas etc
Egg sarnies with plenty of mayo/salad cream in the mix?
Jacket potatoes slathered in butter with a pile of cheese or thickly mayoed coleslaw or both.
Have an appetiser before dinner - some salted nuts & few crisps and a glass of something. That will easily be 300 cals +!