Combine all the ingredients for the fresh herb oil in a 16-ounce mason jar. Give it a little stir. It will be ready to use immediately and will keep in the fridge for several weeks.
Add the mushrooms to a large bowl. Give the herb oil a shake to ensure it’s well mixed, then toss mushrooms with about ½ cup of the herb oil.
Heat a large skillet over medium-high, add the mushrooms and cook until golden brown, seasoning with salt and pepper. When the mushrooms are golden brown remove them from heat and transfer them to a bowl.
Wipe the pan out, return to medium-high heat. Butter both sides of the bread and grill until golden brown, flip, and repeat on the other side.
Top the toast with sauteed mushrooms, crumbled vegan feta cheese, and chives, sprinkle with a little more of the herb oil.