In a small sauce pot, combine the shiitake mushrooms and enough water to cover. Bring to a boil, cover, and cook, stirring occasionally, until softened, 3 to 5 minutes.
While the mushrooms soften, prepare the glass noodles.

Bring a medium [large] sauce pot of water to a boil. Add the glass noodles and cook until just tender, 8 to 10 minutes. Drain and rinse with cold water.
While the water heats and the noodles cook, prepare the stir-fry.

• Trim the root ends from the scallions; cut the scallions crosswise into 2-inch lengths.
• Drain the mushrooms; discard the stems and thinly slice the caps.
• Remove the stem, ribs, and seeds from the bell pepper; thinly slice the pepper.
• Cut a small corner from the steak packaging and drain off any excess liquid. Transfer to a plate; pat dry with a paper towel. Season lightly with salt and pepper.

In a wok or large frying pan over medium-high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Add the steak and cook, stirring occasionally, until browned but not yet cooked through, 2 to 4 minutes. Transfer to a plate.
Stir the scallions and mushrooms into the pan, season with salt and pepper, and cook over medium-high heat until the scallions start to soften, 1 to 2 minutes. Add the bell pepper and carrots and cook, stirring often, until crisp-tender, 3 to 4 minutes. Return the steak and any accumulated juices to the pan and continue cooking, stirring occasionally, until cooked through, 2 to 3 minutes for medium. Stir in the spinach, in batches if needed, and cook until just wilted, 1 to 2 minutes. 

Reduce the heat to low and add the glass noodles and stir-fry blend. Gently toss together and cook until heated through, 1 to 2 minutes. Remove from the heat and season to taste with salt and pepper.

In a large bowl, toss together the glass noodles and stir-fry blend. Add the steak stir-fry and toss to combine. Season to taste with salt and pepper.

Transfer the steak japchae to individual bowls, top with as much sambal oelek as you like, and serve.