In a heavy saucepan, heat the olive oil, then sauté the onions on medium-low heat, adding a pinch of salt. When the onions start to caramelize after 10-15 minutes, add the serrano chili and garlic. Continue to cook another 5 minutes. Stir to keep from burning. Add the ground coriander, tomatoes, and another pinch of salt. If you are not pureeing the sauce, use a box grated to grate the canned tomatoes before adding them to the pan. Add about 1-1 1/2 cups water.
Raise the heat and bring to a simmer, the lower the heat to maintain the simmer. Cook uncovered about 25-35 minutes, or until the tomatoes break down and the sauce reduces. Add more water if you want a thinner consistency.
Meanwhile, warm the remaining oil in a shallow skillet. When the oil shimmers, add the green beans, a pinch of salt, and several grinds of black pepper. Sauté on medium-low heat 10-15 minutes until the beans have softened.
If you want a smooth consistency, blend the tomato sauce with a hand blender. Then add the green beans to the tomato sauce. Add the lemon juice and taste for salt.
Transfer to a serving plate, drizzle with good quality olive oil, and serve alongside rice pilaf.