A Thorny Subject: You may have seen them in grocery stores and wondered how the hell they were used. They are the pale green pads of the prickly pear cactus, sans thorns, known in Spanish as nopales. Nopalitos are nopales cut into thin strips. They are a traditional ingredient in south-of-the-border cuisine yet hard to find in most Mexican restaurants. The "nopalitos en salsa chipotle" ($7.75) at La Mexicana Restaurant [1018 Fairview, (713)521-0963] is exactly what it says: cactus strips cooked in an intense, smoky chipotle sauce. Served with rice and beans, these leaves are mildly tart and tender, though they still retain some crunchiness. Normally prickly pears ooze an okralike slime when cut open, but not these strips. So let's review: no thorns, no slime, intense flavor. Isn't it time to bite into a cactus?