Preheat your oven to 425ºF. Get out 2 rimmed baking sheets. Line 1 with parchment paper.
Toss the sliced brussels sprouts and onions in 2 tablespoons olive oil. Lay in an even layer on the parchment paper lined rimmed baking sheet. Make an opening in the center and lay down the pork tenderloin (I like to tuck the thinner end under to make it an even thickness). Drizzle the pork tenderloin with 1 tablespoon olive oil and 2 teaspoons kosher salt + pepper.
On the other rimmed baking sheet (no parchment paper needed), drizzle 1 tablespoon olive oil on the rimmed baking sheet. Pour on the frozen cauliflower rice, spread in an even layer and break apart any clumps with your hands. Drizzle on another 1 tablespoon olive oil and 1 teaspoon kosher salt.
Put both baking sheets in the oven and roast for 25-30 minutes, until the pork tenderloin has an internal temperature of 145ºF, the brussels are getting crispy on the edges and the cauliflower rice is golden brown.
Remove from the oven, let rest for 2-3 minutes, slice the pork tenderloin and serve warm with brussels sprouts, cauliflower rice and 2 tablespoons of the romesco sauce on top.
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!