Heat a skillet or wok over medium-high until quite hot. Pour in the oils, then add the garlic. Stir for a few seconds, until the garlic softens slightly and starts to become fragrant.
Add the beef strips and stir until the meat loses the redness and is showing only a bit of pink in the center of the strips, 1-2 minutes.
Add the spinach, and toss to begin wilting the leaves.
Add the nam pla, chiles and sugar. Stir to combine and cook down the spinach, about 2 minutes.
If there is more thin sauce than you like, combine the cornstarch with a small amount of water and stir some in to the sauce, allow to boil and thicken. Sometimes I get more or less sauce, depends how much moisture is in the spinach and the meat.