To make the quick pickled cucumbers: In a medium bowl, toss the cucumbers with the vinegar, sugar, and salt until completely coated. Refrigerate until ready to use.
To make the salmon and peas: Preheat the oven to 325°F. Line a baking sheet with parchment.
In a large bowl, whisk together the miso, vinegar, soy sauce, ginger, garlic, brown sugar, and 1 tablespoon of the oil. Place the salmon on the prepared baking sheet. Spoon all but 1 tablespoon of the miso mixture onto the salmon, then brush to coat.
Add the snap peas to the remaining miso mixture in the bowl and toss to coat. Add a splash of water to the miso mixture if it is too thick to coat.
Bake the salmon until it is cooked through and starts to brown on top, about 20 minutes.
While the salmon cooks, in a large skillet, heat the remaining 1 tablespoon oil over high heat. Add the peas and spread them into a single layer. Cook, stirring occasionally, until the peas are slightly browned and charred on all sides, about 4 minutes.
Serve the salmon warm with the snap peas and quick pickles on the side.