If using whole dried mushrooms, roughly chop them into smaller pieces. In a spice grinder or a high-speed blender, combine the dried mushrooms and dried onion flakes. Process until a fine powder is formed. Be careful not to over-process to avoid creating a paste if there's any moisture.Transfer the mushroom and onion powder to a small mixing bowl. Add the nutritional yeast, garlic powder, smoked paprika, fine sea salt, black pepper, mustard powder (if using), and chili flakes (if using).Whisk all ingredients together thoroughly to ensure an even distribution of flavors.Label and store the Savory Umami Seasoning Mix in a clean, airtight jar or container in a cool, dark place. It will retain its potency for up to 6 months.