Season the broth. Add the chicken broth, granulated garlic, granulated onion, dried thyme, and mushroom powder, if using, to a large braiser or heavy-bottomed pot.
Add the noodles. Add the egg noodles and bring to a gentle boil. Cook according to package directions until tender, stirring often to prevent the noodles from sticking to each other or the bottom of the pot. Do not drain.
Make the slurry and thicken the noodles. In a small bowl, whisk the cornstarch with a little cold water until completely smooth. Reduce the heat to low and slowly stir the cornstarch slurry into the noodles. Cook for 1–2 minutes, stirring, until the remaining broth thickens into a creamy sauce that coats the noodles.
Finish. Remove noodles from the heat and stir in the heavy cream. Taste and season with kosher salt and freshly ground black pepper as needed.
Serve. Remove from the heat and serve warm. The sauce will continue to thicken as the noodles sit; stir in a splash of chicken broth if needed to loosen it back up.
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Notes
The broth level matters: Add enough chicken broth to come just barely to the top of the noodles without completely submerging them. This gives the noodles enough liquid to cook while leaving you with the right amount of broth to thicken into that creamy sauce.