Add chilli, vinegar, water, coconut sugar, tomato paste, garlic and salt into a high-speed blender and blend until the chilli is processed into very very small pieces.
Pour into a pan and bring to the boil over high heat. Once boiling, reduce heat to medium-low and let cook for about 10 minutes until about 1/2 the liquid evaporated. Stir every once in a while.
In the meantime, peel and destone avocado and cube into 1/2" pieces. Peel grapefruit out of peel AND SKIN (!) and break into 1/2" pieces. Peel and thinly slice red onion. Set aside.
Brush tortillas with flavorless oil on both sides (I used avocado oil). You can also use your fingers. Make sure to cover the whole surface, also the edges.
Place tortillas on a non-stick baking sheet and turn on the broiler on high. Toast for about 3 minutes on one side and another 3 minutes on the other.
In the meantime, preheat a skillet over medium heat.
Remove sweet-and-sour red chilli sauce from heat and let cool a little.
Take out tostadas from oven and set aside.
Once skillet is hot, add a tiny splash of flavorless oil and shrimps (preferably in their shell), salt, pepper and cook for 2-3 minutes on one side and then 2-3 minutes on the other. Don't crowd the pan. If the pan is small, cook the shrimp in 2-4 batches. Making sure they don't touch each other.
Now build the tostadas by adding arugula, cubed avocado, grapefruit, shrimps and red onion, drizzle with sweet-and-sour red chilli sauce and top with microgreens for decoration.