In a small sauce pot, combine the shiitake mushrooms and enough water to cover. Bring to a boil, remove from the heat, and let stand until softened, 8 to 10 minutes.
While the mushrooms soak, prepare the glass noodles.

Bring a medium 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, start the stir-fry.

• Trim the root ends from the scallions; cut the scallions on the diagonal into 2-inch lengths.
• Remove the stems, ribs, and seeds from the bell peppers; thinly slice the peppers.
• Trim the root ends from the bok choy; cut the bok choy in half lengthwise, then thinly slice crosswise.

• 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 generously with salt and pepper.
• Drain the mushrooms; discard the stems and thinly slice the caps.

In a wok or large frying pan over medium-high heat, warm 1 tablespoon 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 the steak to a plate.
Add the scallions and mushrooms to the pan, season with salt and pepper, and cook, stirring occasionally, until starting to soften, 2 to 3 minutes. Add the bell peppers and carrots and cook, stirring occasionally, until starting to soften, 3 to 4 minutes. Stir in the bok choy and spinach and cook until the bok choy starts to soften and the spinach is just wilted, 1 to 2 minutes. Return the steak and any accumulated juices to the pan and continue cooking until cooked through, 2 to 3 minutes for medium. 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 incorporate. Season to taste with salt and pepper.

Transfer the noodles to individual bowls and serve the sambal oelek, if using, on the side.