Heat a large frying pan over medium heat and cover with olive oil when hot. When the oil is hot, add the garlic and onions. Cook at this heat for 10-15 minutes or until lightly golden and the edges of the garlic bits start turning brown.
Add the spinach and cover the pan to steam for 6-8 minutes. Uncover the pan and cook on medium for another 3-4 minutes or until all the spinach is no longer solid from fridge. Salt to taste.
Add lentils and cook for just one more minute. Salt to taste.
Turn off heat and let sit to cool and then store, covered, in fridge until needed.
To make dough, combine flour, salt, vegetable oil and cold water in a bowl and mix thoroughly until you get a springy and smooth dough. A mixer on medium speed will take about 6-8 minutes.
When done, cover the mixing bowl with cling wrap and refrigerate the dough for 15 minutes.
Preheat oven to 400°F.
Remove both the dough and filling from the fridge.
Start rolling out your dough by taking golf ball-sized dough from the bowl and shaping it into a round ball. Flatten it with the side of your fist and roll it out to make a wrapper. It should be about 5 inches diameter and doesn’t need to be a perfect circle. The dough should not be super thin or thick but it should be slightly thicker in the middle.
When you have your empanada wrapper ready, take a tablespoon of the filling and put in the center. Fold one side of the empanada over the other side so you have somewhat of a half moon. Using a fork, firmly press the edges to seal the empanada.
When done, place on a baking tray. Continue assembling empanadas until they can all go on the tray without touching.