Preheat your oven to 450°F / 230°C.
In a medium bowl, combine the masa harina and salt. Gradually pour in the warm water, mixing with your hands until a soft, pliable dough forms. If it feels crumbly, add more water one teaspoon at a time.
Divide the dough into 14 or 15 equal-sized balls, about the size of a large walnut.
Working one at a time, flatten each ball into a round disk about 3-4 inches (8-10 cm) across. Pinch the edges to create a ¼-inch (½ cm) rim, forming a small, shallow cup.
Arrange the masa shells on a baking sheet and lightly brush the inside of each one with vegetable oil.
Bake for 20 to 25 minutes, or until the edges are golden brown and the shells are firm and almost crisp.
Let the tortita shells cool on the baking sheet for a few minutes until they are cool enough to handle.
Spread a scant teaspoon of green chili sauce evenly inside each baked shell.
In a small bowl, combine the crumbled farmer cheese and feta cheese, mixing them together thoroughly.
Sprinkle the cheese mixture evenly over the chili sauce in each tortita.
Serve immediately as a fantastic appetizer with tequila cocktails or cold beer.