In a medium pot, combine broth and water. Bring to a low simmer over medium heat. Reduce heat to low.
Meanwhile, heat a large straight-sided skillet over medium-high heat until very hot, about 2 minutes. Add half of asparagus in an even layer and cook, turning a few times, until crisp-tender and charred in places, about 4 minutes. Transfer charred asparagus to a cutting board. Repeat with remaining asparagus.
Add lemon slices to skillet and cook, turning halfway through, until lightly charred, about 2 minutes. Transfer to cutting board with asparagus.
Remove skillet from heat. Place onion and shallot in skillet and stir a few times (they will char almost immediately). Add oil and stir to coat.
Return skillet to medium heat. Add a pinch of salt and cook, stirring occasionally, until onion is softened, about 2 minutes. Add garlic and cook, stirring, until fragrant, about 2 minutes.
Stir rice into onion mixture; season with 1 teaspoon salt. Cook, stirring constantly, until rice is lightly toasted, about 2 minutes.
Pour lemon juice into rice mixture and bring to a boil. Cook, stirring constantly, until liquid is evaporated, about 1 minute.
Add a few ladles of warm broth mixture to skillet and cook, stirring, until broth is absorbed. Continue to cook, adding a few ladles of broth as it's absorbed and stirring frequently, until rice is al dente, about 25 minutes.
Remove a few asparagus tips and set aside for serving. Cut remaining asparagus into 1" pieces.