To make the dressing, in a small bowl, whisk together the olive oil, lemon juice, fish sauce, garlic, and salt to taste.
Pour the canola oil into a small, heavy saucepan to a depth of 2 inches and heat to 375°F. While the oil heats, cut the tortillas in half, then stack the 4 halves. With the straight side facing you, cut them vertically into ½"-wide strips. Discard the end pieces, which will be too short.
Working in small batches, add the tortilla strips to the hot oil and deep-fry until they darken in color, about 1 minute, adjusting the heat as needed to maintain the temperature as close to 375°F as possible. Lift them out with a wire-mesh skimmer onto a double thickness of paper towels to drain. Sprinkle with salt while still hot.
Put the sesame seeds in a small, dry skillet and toast over moderate heat, shaking the skillet frequently, until the seeds are golden brown, about 4 minutes. Let cool.
Put the spinach in a large salad bowl. Add the toasted sesame seeds and queso fresco and toss to mix. Add just enough dressing to coat the spinach lightly; you may not need it all. Toss well and taste. The salad will probably need a squeeze of lime juice. Add the fried tortilla strips, toss gently, and serve immediately.
