Heat 2 tablespoons of oil in a large pan over medium heat. Break the spaghetti into shorter pieces and place in the oil. Fry for 5-8 minutes, stirring occasionally, until dark brown—this caramelization is essential for the flavor.
While the spaghetti cooks, peel the garlic cloves and onion, then roughly chop the onion. Open the tomato can and place the garlic, onion, and tomatoes in a blender or food processor. Blend until smooth. Season with a pinch of chili flakes or fresh chili (or cayenne pepper), salt, and pepper to taste.
Halve the avocado lengthwise and remove the pit. Scoop out the flesh and cut into thin slices. Set the pit aside—you will use it when serving to help prevent browning.
Pour the water and sauce over the fried spaghetti. Reduce heat to low and simmer for 30 minutes, stirring every 3-4 minutes, until the spaghetti is al dente and most liquid has been absorbed.
Divide the sopa seca among deep bowls or plates. Top with the avocado slices, crème légère, and the avocado pit in the center. Season with salt and pepper to taste and serve immediately.