In a deep nonstick skillet set over moderate heat, warm the oil until it is hot. Add the mustard seeds and the cumin seeds, and cook, stirring, 1 minute. Add the gingerroot and garlic, and cook, stirring, 1 minute more.
Add the onion and green bell pepper to the seasonings and cook, stirring occasionally, for 5 minutes.
Add the green beans, tomatoes, and salt, and cook, stirring occasionally, for 5 minutes more.
Add the water, bouillon cube, and salt to taste, bring to a boil, stirring, and add couscous. Simmer, stirring occasionally, for 5 to 10 minutes, or until liquid is just absorbed.
Remove from heat, cover, and let stand 5 minutes. Before serving stir in lemon juice and cilantro.