Adjust rack to top position and preheat oven to 450 degrees. Wash and dry all produce.
Halve, peel, and finely dice onion. Trim and halve zucchini lengthwise; scoop out seeds with a spoon and discard.
Heat a drizzle of olive oil in a medium pot over medium-high heat. Add ¾ of the onion; season with salt and pepper. Cook, stirring, until softened, 4-5 minutes. Add rice and Southwest Spice; stir to coat.
Stir in stock concentrate, 1¼ cups water (2¼ cups for 4 servings), and a big pinch of salt. Bring to a boil; cover and reduce to a low simmer. Cook until rice is tender, 15-18 minutes.
Keep covered off heat until ready to use later.
While rice cooks, place zucchini on a baking sheet. Rub all over with a drizzle of olive oil. Season with salt and pepper, then arrange cut sides down.
Roast on top rack until browned and tender, 15-20 minutes.
Meanwhile, dice tomato. Roughly chop cilantro. Zest and halve lime.
In a small bowl, combine tomato, half the cilantro, a squeeze of lime juice to taste, and as much remaining onion as you like. Season with salt and pepper.
In a separate small bowl, combine guacamole, lime zest, and half the sour cream (save the rest for serving). Squeeze in lime juice to taste. Season with salt and pepper. Add water 1 teaspoon at a time until mixture reaches a drizzling consistency.
Once zucchini is tender, remove baking sheet from oven. Heat broiler to high.
Fluff rice with a fork and season with salt and pepper. TIP: Stir in 1 tablespoon butter (2 tablespoons for 4 servings) for extra richness.
Flip over zucchini and stuff with as much rice as will fit (there will be plenty left over). Evenly top with pepper jack.
Broil until cheese is melted and lightly browned, 2-3 minutes. TIP: Watch carefully to avoid burning.
Divide remaining rice between plates and top with stuffed zucchini. Top with avocado crema, pico de gallo, remaining cilantro, and remaining sour cream. Drizzle with hot sauce if desired and serve.