Alex123, fat always has been and always will be your friend.

Good ways to increase it are as follows:
Always use full-fat mayo
1/4 Cup (this is one good-sized handful) of nuts has around 280 calories. That alone would give you quite a boost and make a great snack.
Add olives, cheese and sunflower seeds to your salads
Have a few berries with cream on them.
Sautee your veggies in olive oil and butter.
Eat boiled eggs mashed with butter on them.
Sautee mushrooms in butter to go with the eggs.
Use low-carb bread and make skillet toast occasionally. Skillet toast is buttered on both sides, then fried.
Eat the fats on your meats.

It's pretty easy to up your calories and in the long run much better for your body.