1. Combine soy sauce, lemon juice, cumin, and garlic in a bowl. Add beef and marinate while you gather and prepare the other ingredients.
2. Cut peppers into julienne strips, aout 2 1/2" by 1/4". Cut onion into strips.
3. After beef marinates for about 20 minutes, drain off liquid. Put large saute pan on high heat, until pan just starts to smoke. Add 1 T. oil and add beef. Cook quickly, for about 3-5 minutes.
4. Meanwhile, put another large saute pan on med-high heat. Add 1 T. oil and add onions. Cook for about 3 minutes. Add peppers and season with salt and pepper, to taste. Cook until both are tender, about 4 minutes.
5. Serve both mixtures with warm flour tortillas and accompaniments (see cook's notes).