Preheat the oven to 250 °F and line two large baking sheets with parchment paper.
In a small skillet over medium-high heat, lightly toast the pecans and cashews for 4 minutes, mixing every so often, until fragrant. Roughly chop and transfer to a very large mixing bowl.
In the same large bowl, combine cereal, pretzels, and crispy onions. Mix well and set aside. In a separate bowl, combine the olive oil, melted vegan butter, Worcestershire, tamari, rice vinegar, and sesame oil. Mix well.
In a third small bowl mix together the seasoning blend. Combine the nutritional yeast, onion powder, garlic powder, dried oregano, dried thyme, sweet paprika, cayenne pepper, and black pepper. Mix well.
Slowly drizzle ⅓ of the wet mixture over the bowl of dry cereal ingredients. Using a large spoon or spatula, delicately toss the mixture. Repeat drizzling and mixing until the wet ingredients are thoroughly incorporated with the dry. (Coating everything thoroughly will ensure that the dry seasonings stick to each and every piece.) Sprinkle about ¼ of the seasoning blend over top of the chex mix. Gently toss. Repeat this step until all of the seasoning blend is used.
At this point, taste a little bit to see if it needs any salt. If so, sprinkle a little bit on top, mix, and repeat until you are satisfied. Depending on the cereals and Worcestershire you use, you may not need any at all, so just taste to see.
Divide the chex mix onto the large baking sheets, creating an even thin layer of mix on each sheet (see notes below.) Bake for 20 minutes. Remove the baking sheets and mix it around with a spatula. Return the baking sheets into the oven, rotating the top sheet with the bottom sheet and spinning them around to encourage an even toasting. Continue to bake for another 20 minutes. Remove the baking sheets and mix the chex mix, once again. Return the baking sheets into the oven, rotating again, and bake for 10-15 minutes, depending on how toasty you like it. Remove from the oven and let cool on the baking sheets.
Once the chex mix is completely cool, about 1 hour, transfer the mix into jars and tightly seal with a lid. Enjoy!