Soak cashew nuts overnight in cold water, or do a fast soak by covering them in very hot but not boiling water for 15 minutes. Drain and rinse well, then blend with ½ cup of water until smooth.
Heat 1 tablespoon of olive oil in a frypan (skillet), add onion and cook until golden and softened.
Add baby spinach, stir and cook until spinach has wilted.
Add chopped artichoke hearts and cook for another 1-2 minutes.
Remove spinach and artichoke mixture to a bowl, then add lemon zest, lemon juice, blended cashews, salt and a good grind of black pepper. Stir to combine. Taste and adjust seasoning as needed.
Brush a sheet of filo pastry lightly with olive oil, stack another on top and brush with more oil. Arrange about 2 dessert spoons of filling along the long edge in front of you. Roll to create a long snake, then twirl into a snail shape. Repeat with remaining pastry and filling until you have 7-8 snails.
Pack the borek snails closely into a cast iron or other baking dish that has been brushed with oil, brush the top of the borek with a little more oil and sprinkle with sesame seeds.