In a medium saucepan of boiling water cook the beans, uncovered, until crisp-tender, about 6 minutes.
Drain and rinse with cold running water to stop the cooking.
Reserve.
In a large nonstick skillet, heat the olive oil over medium heat and cook the onion, mushrooms, and garlic, stirring frequently, until the mushrooms start to brown, 6 to 8 minutes.
Add the reserved green beans and salt.
Cook, stirring, until completely heated through, 3 to 4 minutes.
Transfer to a serving bowl.
Scatter the diced tomatoes on top.