Arrange a steam basket in a large pot (4 qt. or larger), and add about an inch of water to the bottom of the pot. Place the chopped cauliflower florets in the steam basket, bring the water to a boil, then cover with a lid and reduce the heat to a simmer for about 7 minutes. You want the cauliflower to be fork-tender, but not mushy!
Remove the steamed cauliflower, drain the cooking water, and return the cauliflower to the large pot. Add the fresh spinach and sliced tomatoes immediately, allowing the heat from the cooked cauliflower to gently wilt the spinach and soften the tomatoes.
Add in the olive oil, lemon juice, minced garlic, fresh basil and oregano leaves, sliced olives (if using), plus a generous sprinkling of sea salt and pepper. Toss well to coat.
Allow the dish to cool before adding the crumbled feta, to avoid melting the cheese. Serve immediately at room temperature, or allow to marinate in the fridge for a later meal. I personally prefer this dish served at room temperature for best flavor and texture, but it's delicious directly from the fridge, as well!
*If you don't own a steam basket, you can simply place the cauliflower directly in the pot with an inch of water (no need to cover the cauliflower with water completely). Bring to a boil, then reduce the heat and cover the pot to allow to "steam" for 7 minutes and drain. Some of the nutrition may be lost through contact with the water, but it's nothing to worry about!