Heat a large skillet with sides over medium heat. Add olive oil and when the oil is hot, add shallot, garlic, crushed red pepper and cook, stirring, until tender, about 4 minutes.
Add the diced tomatoes, kosher salt, and pepper and cook, stirring, until the tomatoes soften, about 2 more minutes.
Add the arrowroot and whisk into the sauteed veggies until well combined.
Add 1/2 of the baby spinach (or as much as you can fit into your skillet) and cook, tossing occasionally, until wilted down. Continue adding the spinach into the skillet, tossing gently, until just wilted. You will likely have to do this in a few batches.
Add the nutritional yeast, coconut milk, chicken broth, lemon juice and stir to combine. Bring to a simmer and cook, stirring often, until the sauce thickens a bit, 4 to 5 minutes. Reduce heat and keep warm, tossing occasionally, while you cook the fish.