Steam the cauliflower florets until tender or blanche in salted boiling water, about 3 or 4 minutes. Drain thoroughly in a colander and run under cold water to stop the cooking process. Allow to dry.
To make the dressing: combine the capers, mustard, garlic, vinegar, ½ teaspoon sea salt and some black pepper. Whisk briskly while slowly adding the 1/3 cup oil. Taste for seasoning.
Heat 1 Tablespoon of olive oil over medium heat in a large skillet. Add the cauliflower and sauté until hot and just starting to brown. Transfer to a serving bowl. While the cauliflower is still hot, add the dressing, dill, spinach and tomatoes. Stir together well, then taste for seasoning.
Serve warm or at room temperature.