Prepare thyme crisps (if making): Preheat oven to 350ºF. In a large bowl, toss all crisp ingredients. Place 6 1-tbsp mounds of mixture, evenly spaced apart, on a large parchment-lined baking sheet. Pat the mounds, using your fingers, into 3½- to 4-inch rounds.
Bake for 8 minutes until crisps are light brown. Set aside to cool on sheet for 5 minutes, then use a metal spatula to transfer to a wire rack to cool completely. Using a cool baking sheet, repeat to cook remaining crisps.
Meanwhile, fill a small pot halfway with water and bring to a boil. Add spinach and cook until thawed, about 2 minutes. Add watercress and continue to cook just 2 minutes more. Drain and cool quickly under cold running water. Continue to drain in strainer, squeezing out as much water from spinach and watercress as possible. (NOTE: If using fresh spinach instead of frozen, blanch for only 2 minutes at the same time as watercress.)
In a large saucepan on medium-low, heat oil. Add onion and 1/4 tsp salt and cook for 6 to 7 minutes, stirring occasionally, until onion is soft and beginning to turn golden. Add garlic, stir, and cook until fragrant, 1 minute. Add broth, increase heat to high and bring to a simmer; remove from heat.
To a blender, add spinach, watercress, mint and stock mixture. Blend until smooth. Add butter, lemon juice, pepper and remaining 1 tsp salt; continue to blend until fully incorporated. Season to taste with additional salt and pepper. Serve soup in individual bowls. If using, add a dollop of crème fraîche and a few cheesy thyme crisps.