Preheat the oven to 450 F, using the convection roast setting, and bring a large pot of water to boil.
Add the potatoes to the pot of boiling water, along with 1 teaspoon of the salt and the baking soda. Bring it back to a boil, and cook on medium-high for 5 minutes, or until the potatoes are slightly fork-tender.
Drain the potatoes, dry them lightly with a tea towel or paper towel, and place in a large bowl. Add the remaining salt, avocado oil, garlic powder, thyme, and pepper. Toss to coat, and place them face down on a parchment-lined baking sheet.
Bake in the oven for 15 minutes, flip them over, and broil for 5 minutes. Top the potatoes with the watercress before serving.