Using a strainer, rinse rice until water runs clear. Add rice to the boiling water, then reduce heat to low. Cover and cook until rice is tender and liquid is absorbed, 12-14 min. Remove from heat. Set aside, still covered.
Meanwhile, peel, then cut onion into ¼-inch pieces. Roughly chop cilantro. Cut tomato into ¼-inch pieces. Zest, then juice lime.
Meanwhile, heat a large non-stick pan over medium-high heat. When hot, add 1 tbsp (2 tbsp) oil, then half the corn (use all for 4 ppl). Cook, stirring occasionally, until corn starts to char, 2 min. Add onions. Cook, stirring occasionally, until tender, 3-4 min. Season with salt and pepper. Remove from heat. Transfer veggies to a medium bowl to cool slightly. Carefully wipe the pan clean.
Using a strainer, drain and rinse shrimp, then pat dry with paper towels. Season with pepper and ½ tsp (1 tsp) garlic salt. Reheat the same pan over medium-high. When hot, add ½ tbsp (1 tbsp) oil, then shrimp. Cook, stirring occasionally, until shrimp just turn pink, 2-3 min.** Remove from heat.
Meanwhile, add tomatoes, half the cilantro, half the lime zest, ½ tsp (1 tsp) sugar and ½ tbsp (1 tbsp) lime juice to the medium bowl with veggies. Season with salt and pepper, then stir to combine.
Fluff rice with a fork, then add remaining lime zest and 1 tbsp (2 tbsp) butter, stirring until melted, 1 min. Divide rice between bowls. Top with salsa and shrimp. Drizzle chipotle sauce over shrimp. Sprinkle with remaining cilantro.
If you've opted to get shrimp, using a strainer, drain and rinse shrimp, then pat dry with paper towels. Season with salt and pepper. Reheat the same pan (from step 3) over medium-high. When hot, add ½ tbsp (1 tbsp) oil, then shrimp. Cook, stirring occasionally, until shrimp just turn pink, 2-3 min.** Remove from heat.