How to make aluminium monostearate?
Aluminium hydroxide isn't a partilarly reactive or soluble compound, so you probably don't want to directly mix it with stearic acid; also, you'd likely obtain a tristearate this way.
According to a procedure described in US3056819A patent, a synthetic path would be to obtain aluminum dihydroxychloride $\ce{Al(OH)2Cl}$ first, and then add stearic acid and sodium hydroxide (please disregard full stops after units, this isn't a standard notation these days):


  (1) 2.7 g. (0.1 mole) of metallic aluminium is dissolved in 50 cc. of 2N-HCl (0.1 mole) under heating to obtain a solution of a basic aluminium chloride whose formula is $\ce{Al(OH)2Cl}.$
  
  (2) To an aqueous solution of 47.58 g (0.1 mole) of potassium alum $(\ce{AlK(SO4)·12H2O})$ is added a solution of sodium bicarbonate to form aluminium hydroxide.
  The aluminium hydroxide is dissolved in 50 cc. of 2N-HCl with stirring and the solution is heated for 1 hour at 100 °C. to give a solution of a basic aluminium chloride whose formula is $\ce{Al(OH)2Cl}.$
  
  (3) To a solution of 24.2 g. (0.1 mole) of hydrous aluminium chloride in 100 cc. of water is added gradually 16.8 g. (0.2 mole) of sodium bicarbonate with stirring and the solution is heated for 1 hour at 100° C. on a water-bath.
  
  Any of the solutions of the basic aluminium chloride produced by the above three methods is diluted with water to 200 cc. and added to a solution of 28.45 g. (0.1 mole) of stearic acid and 4 g. of sodium hydroxide (0.1 mole) in 1000 cc. of water, and the mixture is heated on a water bath until the product precipitates.
  After washing with water to remove chlorine ion, the product is extracted with acetone in Soxhlet's extractor to remove free stearic acid, obtaining 33 g. of aluminium monostearate whose formula is $\ce{Al(OH)2(C17H35COO)}.$


References


Hideo, T. Aluminium Soaps and Their Production. US3056819A, October 2, 1962. (PDF)