Enphase Energy (ENPH) had the highest return in July 2019 by a mid-cap US stock, returning 49.7%.

ASSET	MONTH	% RETURN
Enphase Energy (ENPH)	July 2019	49.73%
Sellas Life Sciences (SLS)	July 2019	41.67%
Fortuna Silver Mines (FSM)	July 2019	33.33%
Elastic (ESTC)	July 2019	30.55%
Nio Inc Class A ADR (NIO)	July 2019	29.96%
Spyre Therapeutics (SYRE)	July 2019	29.86%
Mattel (MAT)	July 2019	29.2%
Immunitybio (IBRX)	July 2019	28.97%
Penguin Solutions (PENG)	July 2019	28.28%
Silvercorp Metals (SVM)	July 2019	27.92%
Indivior PLC Ordinary Shares (INDV)	July 2019	25.24%
First Majestic Silver (AG)	July 2019	23.79%
FTI Consulting (FCN)	July 2019	23.45%
M/I Homes (MHO)	July 2019	23.11%
HealthEquity (HQY)	July 2019	23.07%
Extreme Networks (EXTR)	July 2019	21.86%
Meritage (MTH)	July 2019	21.56%
Vista Energy, SAB de CV (VIST)	July 2019	21.45%
Navios Maritime Partners LP Unit (NMM)	July 2019	21.38%
OneMain (OMF)	July 2019	21.23%
Ringcentral (RNG)	July 2019	20.8%
Impinj (PI)	July 2019	20.67%
AMC Entertainment (AMC)	July 2019	20.1%
Huron Consulting (HURN)	July 2019	19.9%
Comstock Resources (CRK)	July 2019	19.61%

Mid-cap stocks are treated as having a market cap between $2B and $10B.