Preheat your oven to 425ºF.
Line a rimmed baking sheet with parchment paper (for easy cleanup) and set in its wire rack. Lay the marinated chicken thighs in an even layer on the wire rack. Roast for 30 minutes, or until crispy and caramelized on the edges.
Get another rimmed baking sheet, and drizzle it with 1 tablespoon olive oil. Lightly pound the bag of frozen cauliflower rice to break up and clumps. Lay the cauliflower rice in an even layer, and break up any other clumps you see. Drizzle it with 1 more tablespoon olive oil, then season with kosher salt (about 1/2 teaspoon). Roast for 25 minutes, until it is starting to turn golden brown on the edges. (Depending on your oven, you may need to turn the cauliflower rice once while roasting. My oven cooks really evenly, so no need for me to flip mine.)
Remove the chicken and cauliflower rice from the oven. Chop the chicken thighs into bite size pieces.
Assemble each bowl with a big handful of spinach and top with 1/4th of the cauliflower rice, 1/4th of the chicken shawarma, the tomatoes and 2 tablespoons tzatziki. Season the tomatoes with kosher salt.
Leftovers will keep in an airtight container refrigerated for at least 5 days, or frozen for at least 6 months.
As always, lots of tips, FAQs and substitution info in the Recipe Notes below!