Add olive oil to a medium pot over medium-high heat. Once heated, add diced onion and cook for 5 minutes, until translucent. Add minced garlic and cook for 30 seconds to 1 minute, until fragrant.
Reduce heat to medium and add the riced cauliflower. Cook for about 5 minutes, stirring occasionally, until it's released and cooked off most of its moisture.
Add the diced tomatoes, fajita spice, kosher salt, and black pepper. Stir and cook for 7 to 10 minutes, until the liquid has mostly evaporated and soaked into the rice.
Create a well in the center of the pot and add the tomato paste. Let it cook directly in the well for about 1 minute before stirring it through. Cook for 5 more minutes, allowing the flavors to combine.
Remove from heat, squeeze in the lime juice and stir in cilantro, if using. Serve.