Mix cornstarch and 1 tablespoon of water in a small bowl until the cornstarch dissolves completely. Your slurry should be completely smooth. Otherwise, you risk lumps of cornstarch in your sauce. Set aside.

Add remaining water, flour, brown sugar, soy sauce, salt, and pepper to a saucepan and cook over medium-high heat until it comes to a boil. Whisk occasionally until the brown sugar has completely dissolved.

Still whisking, pour in the cornstarch slurry. Reduce the heat to medium and continue whisking for 5 minutes until the sauce is thick.

Once thickened, reduce the heat to low and add shallots, garlic, and chilis to the saucepan. Continue to simmer for 2 minutes.

Remove sauce from the heat and serve immediately.