Core and halve tomatoes. Remove seeds (either scoop them out with a spoon or hold a half in your hand and squeeze out the seeds) and cut tomatoes into bite-size pieces. Put tomatoes in a large bowl.
Remove stem and seeds from chile. Finely chop chile and add to tomatoes. Set aside.
In a small bowl, whisk together olive oil, lime juice, 1/2 tsp. salt, and mustard until emulsified.
Drizzle dressing over tomatoes and chiles and toss gently to combine. Add more salt to taste if you like. Serve at room temperature.
Note: Nutritional analysis is per serving.