Make the tahini sauce: Whisk 2 tablespoons warm water, the tahini, lemon zest and lemon juice, 1/4 teaspoon salt and a few grinds of pepper in a small bowl until smooth. (It will look curdled and broken at first but keep whisking.)
Heat 2 tablespoons vegetable oil in a large nonstick skillet over medium heat. Add half of the green beans and cook, stirring occasionally, until they start browning, 6 minutes. Increase the heat to medium high and cook until wrinkled and blistered in spots, 4 to 6 more minutes. Add 2 teaspoons garlic, 1/2 teaspoon each rosemary and salt and cook, stirring, until the garlic is golden, 1 minute. Transfer the green beans to a platter; wipe out the pan and repeat with the remaining 2 tablespoons vegetable oil, green beans, garlic, rosemary and 1/2 teaspoon salt.
Before serving, drizzle the green beans with the tahini sauce. Sprinkle with the parsley and grate some more lemon zest on top.